I was putting together a PCB for a lamp retrofit, and I needed to know the board would actually fit before I ordered the real thing - the screw holes had to line up and the board had to clear the lamp's metal shell. Shaping paper by hand wasn't going to give me sub-millimeter accuracy, so instead I made a cardboard mockup of the PCB itself.

KiCad lets you export the board as a 3D model straight from File > Export, and switching it to STL gives you a printable mesh. I ran that off on my 3D printer for a rigid test-fit board, and put a thinner outline through my laser cutter to check the mounting holes.

It caught a couple of fit problems before they cost me a re-order. If you design PCBs, printing a quick mockup beats trusting your eyes every time. There's a fuller write-up on my website.