I'm building picture frames at the moment, and ran out of corner clamps. What is slightly worse is, that my local DIY store is also out now.

Therefore I came up with a simple design I could print. I my humble opinion, these works amazingly nice. Just print four of these and use a rubber band to pull the frame together.

I made them quite big so they provide some support making it easy to get the angle right. And with the groove under the miter joint, there is no chance you glue the frame to the corner clamp.

This design will support up to about 50mm broad frame, but it should be easy to scale if you need something else. I made the edge of the corner 15mm high. The frames I'm building are 21mm high so I have free space to attach a glue clamp