Picture frame brace to hold frame in place when gluing.
Print 4 braces (about 2.5 hours each) and secure with a ratchet strap to clamp the frame.

I printed with 15% infill. I recommend 20 or 25%.

The braces in my photo are version 1. The STL is been updated to round the corners on each end. After printing the original version it needed to be sanded around the corners for a good fit to the frame. The new version will negate the need to sand after printing.