Parametric slide-together picture frames. To assemble (note that 3 out of 4 of the corners are trapezoidal mating tracks, while the fourth is rectangular):

• Slide 3 edges together (only use trapezoidal tracks; leave an edge with a rectangular track aside for now)
• Insert the piece to be framed into the inner slots
• Slide the remaining frame into place. It may be necessary to glue the square track depending on the dimensions and orientation.

I find the frame holds itself together just fine without glue.

I print the 4-edge groups diagonally at 45° so that I can accommodate the largest frame possible on my build plate.

Parametric Fusion 360 file