This frame by MadMax3D is friggin' awesome.
https://makerworld.com/en/models/2751321-parametric-picture-frame?from=search#profileId-3052706

I made this connector piece because I wanted to connect 4 frames together to use as a backdrop for my statues. Works perfectly. Connecting 4 frames takes 8 of these and about 45 minutes to print with 0.4mm nozzle and default print settings.