This is a wholly new design inspired by Hexakon's ratchet strap corner brace. I had some trouble printing that model, so I created my own from scratch using Fusion 360.

Print without supports. There is some overhang on the top edge, but a printer with dialed in bridging settings should have no problems with it.

The parameterized Fusion 360 source file is included if you wish to generate additional sizes.

Print quantities:

60 degree - 3 (triangle)
90 degree - 4 (square or rectangle)
120 degree - 6 (hexagon)
135 degree - 8 (octagon)