Customizable mitre box for extrusions (e.g., OpenBeam)

Simple mitre box to nicely cut OpenBeam (or, possibly, other extrusions).

The idea for the screws doubling as guide pegs comes from [Terence Tam's mitre box](http://www.openbeamusa.com/blog/2012/08/06/howto-cutting-openbeam-hand-tools) (not on Thingiverse), however that was too elaborate (esp. the need for a hold-down clamp). I found a simpler one here (see sources), but wanted to clamp it down. Hence this, which uses regular M3 screws as guide pegs, and < $5 ratcheting clamps. I was able to get almost perfectly repeatable cuts; YMMV though.