So I recently tried queen rearing using little polystyrene mating nucs ordered from China. These each have three tiny plastic frames, which come in three pieces each (a top bar and two sides). Unfortunately, the foundation kept falling out of these for me at the worst possible times, so I've designed these experimental frames in the same size to see whether I can print something sturdier.

If you print these with the top bar downwards, they shouldn't require supports.

NOTE: These are still experimental, and it will be some time before I can test it properly. The hole in the top is very much a guesstimated size, and I will tweak for the Nicot cups I use in a couple of months when I test it.

The models include a top-slotted frame (slide in a sheet of foundation from the top), and an experimental version with integrated plastic foundation, which should hopefully also add structural rigidity. Note the plastic version will require a beeswax coating, and I have no idea whether the bees will accept/reject any particular plastics.

If you grab the OpenSCAD files, you can change the width, height, etc. I haven't rigged it for Customiser yet though. To build from OpenSCAD, you will need my honeycomb library from https://www.thingiverse.com/thing:4025172 .

EDIT 1: Slightly increased size - had allowed too much tolerance so was loose in nuc. Increased width and height to full 105, increased length of tabs.

EDIT 2: Updated STL with foundation after a correction to the underlying honeycomb library (cell shape issue).