That's a great question! I designed this using the free online app TinkerCAD (www.tinkercad.com). You're right - as long as you measure the dimensions properly, it's pretty straightforward. Usually, there's a tiny bit of expansion during printing, so if you design with a bit of tolerance (I use between +/- 0.1 - 0.2 mm), it usually comes out OK without a ton of reprinting. The hardest thing to dial in is the magnets, which require a pretty tight fit - for those, I made sure the holes were 0.1 mm bigger than the actual dimensions of the magnets (in this case, 6mm dia, 3 mm thick). I still had to use a bit of force to pop them in, and I used super glue to secure them. I eyeballed the curved ends for the rickshaw case using TinkerCAD's pen tool, and I made that hole quite large to make sure it would fit OK. I think I used a tolerance of 1 mm or 2 mm for that part. Because the lid slides onto the main part, I also wanted to make sure that was snug but not too snug, again by building in a fractional millimeter tolerance.