Use two Teenage Engineering Pocket Operators back-to-back simultaneously. This minimal case allows use at the same time and a 6-inch right angle stereo connector fits perfectly between the two for the sync signal.

The brackets are symmetrical so left/right doesn't matter.

You need an M3x10 mm screw to clamp the left/right sides together, but I think any M3 between 8 and 15 mm long would work fine.

I'm using it for the PO-20 Arcade and the PO-14 Sub, but any Pocket Operator should fit perfect.

I used this cable from Amazon for the sync (amazon.com/dp/B01L0YPVH6).

Designed in blender.

Changes:

v5: Added inner & outer chamfer to corner to make easier to print.

Print instructions

No supports. Upped perimeters to 4, but otherwise default PLA print.