Have an old skateboard laying around?
Try building this super easy skateboard shelf. The brackets use the holes from the skateboard and even the hardware for the trucks. The predrilled holes are 75mm from the wall/edge of board and the second hole is 40mm from that.
I'm sure it will work with all standard sized skateboards and more if you drill the holes to fit.
I did the infill at 100% and it came out pretty beefy, to save time and filament, I'd print at 50%-75% and it should still be pretty strong.
If you print and like this thing please consider sending my a dollar of Karma. If not, no biggie, I'll send you good Karma anyways!
Mine warped a little on the tips, didn't hurt anything and is still pretty strong.