Build Play per flank. Provisional default – check it with the test corner.
Print a test corner first
Three rail pieces (60 mm and 60 mm with miter, 27 mm straight), one corner snap wedge, one straight snap wedge and one picture holder – with your values. Tests the corner, straight joint and picture holder: fit, miter and whether the picture holder snaps in and holds before the full frame prints for hours.
Connectors and picture holders can be printed in a tougher filament, e.g. Basic PLA with a wood PLA frame. Join the straight joints first, then insert the picture. Picture holders: insert the picture, then the holders – pull them out to change it.