I wanted a square tool to draw right angle lines on my whiteboard. Couldn't find one so I just designed one myself :-)

You'll need:
1x 20x2mm aluminium flat bar of your desired length
1x m4x12 countersunk head screw
1x washer
1x m4 nut, I used an cap nut

Drill a hole into the flat bar, secure it with the screw and you're good to go :-)