Simple but effective bow square, use with an elastic band or cable tie to hold string contact. 

print 2x clips , I used a 2mm brim to help hold these, square itself prints flat with no brim or supports, I found 30% infill to be plenty.

The clips then slot into the main body and are secured with glue or a quick plastic weld with soldering iron.

Printed with .6 Nozzle on sidewinder x2.