Not for the faint of heart, but here are all the parts needed to make your own custom 3D printer enclosure. My Prusa MK3S+ and filament drier live in my basement, which is always cold, so I wanted trap the heat for better prints (i.e. reduced warping), keep dust off the printer, and add a fan and HEPA filter for fine particulates and VOCs.  There weren't any off-the-shelf enclosures that would fit all of these requirements, so I decided to design PETG corner braces and TPU gaskets that would work with 0.93" acrylic sheets from the home improvement store. I cut out access doors and installed under-cabinet LED lighting, and it works and looks great:

The first things I designed were the corner brace pieces that join the acrylic sheets together. The acrylic sheets were cut into two sizes, 18" X 32" and 18" X 18". (On the corner brace pieces, there is necessarily support material, at least how I printed them on one ‘axis’, so the left-over bumps from removing the supports can make the fit more snug on that ‘axis’.)

These corner pieces also act a drill guides for drilling the holes during final assembly and are meant for these #4 self-tapping screws:  https://www.mcmaster.com/90417A117/ Since the fit is snug, sometimes requiring a light tap of a hammer to fully seat, I recommend cutting access doors before final assembly.

The other key component was the flexible TPU gasket pieces that fill the spaces in between the rigid corner pieces. Being flexible allows them to be pushed into place during final assembly, and they can be cut to length as needed for passing cords/cables through. I have two different lengths in the design, one for going between the ‘mid-corner’ one for going between the ‘top-corner’ pieces. So it takes two gasket pieces to fill each section.

I cut access doors on the front and side, and used some hinges from the home improvement center. But I couldn't find any appropriate latches, so I designed some that work with small magnets that you can get here: https://a.co/d/iU1vmuz

One magnet is bolted to the enclosure, and the other sits inside the pocket in the front handle piece, so they ‘snap’ together when the latch is closed. The front and back pieces of the handles are ‘keyed’ together and are held together with a screw:

The final component was to add a fan and HEPA filter, so I found a 3-speed AV enclosure fan here: https://a.co/d/5V63bMg 

And a HEPA filter that was close to the correct size: https://a.co/d/5AjNYm0

The filter was a bit larger, so I designed a friction-fit adapter between the two:

With the doors closed, there is a slight leak around their perimeters, so the fan gradually brings in outside air and probably maintains a slight negative pressure inside the enclosure. I don't smell the typical PLA or PETG odor in the basement when printing, so the filtering appears to be working. The temps can get up into the 80 deg F range during a long PETG print, so I can crack open the doors to let more air in if I want to lower the temps. 

So with the LED lighting and fully-assembled, it works great and has been worth the effort of putting together.