I needed such parts to make a frame with different angles so I designed my own.
The angles are 90, 120 and 135 degrees so it will fit for 4, 6 and 8 sided frames.
The corners are not touching and have a hole that the glue can trip all the way down.
Print with high infill and enough perimeters from a solid material (petg, asa, abs, nylon).
I´ve designed it in fusion360 with parameters - so if you need angles for 3,5,7,9 sided frames let me know and ill upload those too.