Update Update: I've redesigned aspects of every part of this box in order to be more robust. I've been riding around with it on my bike for a couple of days now, and it holds tight even over very rough roads, riding down curbs, and so on. There are more parts now, but such is the price we pay for robustness.

I wanted a place to store my bike road tools, so I made this saddle box for them. This is custom-fitted to my bike and seat, and so I can't guarantee it will fit yours.

You'll need additional hardware to assemble this:

• Two M4 x 10mm x 6mm brass screw inserts
• Two M4 bolts, 6 - 10mm long
• Three M3 x 4mm x 5mm brass screw inserts
• Three M3 bolts, 6mm long
• 1mm rubber sheeting

Print:

• one each of the strap and top
• one of the bottom. This will require the use of supports, on the build plate only
• three latches and three guards

Assembly instructions:

• Install the M4 brass inserts into the holes on the sides of the strap (see pic).
• Install the M3 brass inserts into the holes in the three spindles on the sides of the bottom piece. This is not shown in the pics (sorry, I didn't take new pictures after this modification.)
• Line the grooves in the strap and top piece with 1mm rubber.
• Line the large groove on the front of the body with 1mm rubber. You'll want to attach this with an appropriate adhesive.
• The three latches snap onto the spindles. These take a bit of force to get them on, but once seated properly allow the latches to rotate easily.
• For each of the three guards, insert an M3 bolt in the hole in the guard, and screw the assembly into the brass inserts in the spindles. These keep the latches from popping off when riding over very hard bumps. If you don't need that level of robustness, you can leave these off entirely.
• Place the strap on top of the support bars under the bike seat so that the support bars rest completely in the grooves.
• Slide the top piece up from below the support bars so that the mounting block mates with the fingers of the strap.
• Attach the top piece to the strap using 2 10mm M4 bolts. (I actually used 6mm bolts, but I wouldn't go much shorter than that.)

The bottom is attached and removed from the bike using the latches. Each latch has a lock tab that allows the installation of a small lock. This won't give you great security, but might be enough to prevent casual theft.

Note There is a groove on the rim of the top piece that is intended to accept a gasket to improve water resistance, and a Gasket.stl file to print the gasket with flexible filament. In practice, however, this is unnecessary. I'm including this just in case you want to make this box hurricane-proof.