I had an old bed frame that had a lot of wooden slats (~4.5mm thick) that I didn't want to throw out so I made these simple clips to hold the wood in a rectangle form to make a stacked planter bed. If you need them a different size you should be able to scale them in your slicer.

I strongly recommend ABS or ASA for outdoor reliability and some flex for the pinch to work. PETG would probably be too stiff.