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 (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.