This is my entry for 3D Printing Nerd's design contest. It's a simple spool holder designed to clip onto a 1x4 and hold a spool of filament.

In testing it I found that I wasn't able to break it. It would fall off the board with roughly 40lbs of downward force (a guess, since I don't have a crane scale handy) but it would not break at all. 

My print settings for the above test were 5 shells and 75% infill. It obviously could be printed quite a bit less sturdily and do it's job of holding a 5lb spool of filament quite well.