My main pair of glasses folds down fairly flat, but has the hooks on the legs stick down a lot. I was unable to find a case that would fit the legs that wasn't also stupidly thick and fit nicely in a pocket, so I made this. It can be used as a hard case for anything that fits inside.

Inside dimensions:

140mm wide * 68mm deep * 25mm total height

Material:

PET-G or PLA

Any material that can keep the dimensions needed for the hinge can be used. I have included a test piece so you can see if your printer can print the part with proper tolerances to use a piece of filament as a hinge. 

Supports:

No

Should not be required, but if the outside corners come out poorly add a small bit at each to keep them in place

Print Settings

Infill:

20%

Mainly for the magnet mounts

Number of perimeters:

3

I recommend using 3 perimeters with a 0.4mm nozzle to make the sides of the case nearly solid.

Required hardware/tools:

~13cm extra 1.75mm filament for hinge pin

4x 5*1mm magnets

Gel Superglue

Lighter

Flat metal tool to smoosh softened filament Ex: scraper included with most printers 

optional Microfiber cloth 18*18cm (7*7in)

Hobby knife to cut cloth

Small, sharp scissors to trim the cloth

Assembly:

I recommend printing the Hinge Test piece first to make sure your filament will fit in the hinge. 

1. Check to make sure nothing is blocking any of the hinge holes. 
2. Line up the holes in the hinges so that the magnet wells will line up with each other, then push your filament through the hole. Leave about 10 to 15mm sticking out total so you have enough to melt into end caps on the hinge. 
3. Once you have checked that the hinge operates smoothly, melt the ends to make them stay.
   1. Push the hinge filament so about 3 to 5mm of it is sticking out. 
   2. Take the lighter and ignite it away from the part, then hold the flame over the filament for about 2-3 seconds. 
   3. If you see the filament catch fire, immediately remove it from the flame. 
   4. Once the filament is softened, take your flat metal tool and press the filament into the end of the hinge. It should create a small cap to keep the hinge in place similar to what is shown in the second to last picture. 
   5. If it did not soften enough to flatten it hit it with the lighter for another second or 2. If the filament melted completely, I recommend quickly pushing from the other end so you can remove that piece and replace it. 
   6. Once you have the first side done, cut the other side to about 3 to 5mm and repeat steps 3-2 to 3-5 as needed on that side.
4. Add the magnets to the small holes inside the case. Put a small drop of glue in the hole, then press the magnets into the holes. Make sure that the magnet's polarity will pull the case closed.
5. If you want to add a layer of microfiber cloth to keep glasses from getting scratched, cut the 7*7 inch piece of microfiber cloth in half so you have 2 rectangular pieces that are 7*3.5in. For all glue applications, you may want to spread it with a toothpick or other tool to help keep it from soaking through the cloth. This is also why I say you should use gel-type superglue. The last picture here is a diagram showing the glue should be applied in each step.
   1. Apply a small line of glue to the straight inside edge of the case on the hinge side, making sure not to get any on or in the hinge.
   2. Take one of the pieces and hold it so the 7-inch side lines up with the hinge side of one half of the case. Make sure it is centered on that side. Lightly press the edge into the glue, making sure not to get any glue on the inside of the cloth.  Always allow the glue to set enough that gently pulling it won't pull it off of the glue. 
   3. Apply a line of glue near the bottom of the curve on the side you have already glued the cloth to. Gently pull the cloth taut and press it into the glue. You may want to spread the glue with a toothpick or other tool to make sure the cloth sticks to the entire curve.
   4. Apply a spread of glue in the bottom of the case, staying about 20mm from the curved edges. This can be a simple swirl to cover a general area. Gently pull the cloth taut and push it from the center at the bottom of the curve outward to the edges to keep the cloth taut.
   5. Apply a line of glue to the bottom edge of the curve on the magnet side of the case. gently pull the cloth taut over the glue, then press it into the glue starting from the center.
   6. Apply a line of glue to the top inside edge between the magnet mounts, and pull the cloth taut over it. Press it into the glue, making sure to press the cloth into the corners where the magnet mounts are Any overhang here is fine, it will be cut off later.
   7. Apply a light spread of glue to the vertical part of one of the magnet mounts that is still exposed. Gently pull the cloth around the post, doing your best to avoid leaving any creases or folds in the cloth inside the case except for the corners of the magnet mount.
   8. Apply a line of glue to the bottom edge of the curve on one of the short sides. Press the cloth into it starting from the side with the magnet mount, keeping the cloth taut across the bottom.
   9. Apply a line to the straight inside top edge on the same side you just glued. Again press the cloth from the magnet mount to the hinge side. This should create a fold in the corner nearest the hinge.
   10. If the fold is small enough, you can just glue it down. I recommend carefully cutting the cloth with a hobby knife from the bottom curve up to the top edge of the corner, then gluing down any bits from the hinge side down first to the case, then overlaying anything from the side over that and using a small dot of glue to keep it in place.
   11. Repeat steps 5-7 to 5-10 for the other edge.
   12. Trim any excess cloth that extends above the edges of the case. There should be a small bit on the magnet side of the case, and on the short sides. There may be some from the hinge-side corners depending on how you cut it. My example used a cloth that was slightly too narrow, so the side edges don't reach the top.
   13. Repeat steps 5-1 to 5-12 for the other half of the case.

You should now have a low-profile case to use for your glasses or any other small things you may need to carry.