This is for a cheap Temu marble run that uses smaller (12mm) marbles.

Came with small trays that only held like 8 before jamming.

Should work fine for larger marble runs with some scaling.

I made it 2 units tall so I could make a larger exit slot without weakening it, but I can adjust it to make it 1 unit tall upon request.

The return is sloped to prevent jams and the tray has a slight slope so marbles should all make their way to the front.

I printed with a 1mm nozzle and the collar fits the rest of the pieces snugly, but if it's too tight or too loose, with your setup you may need to scale X and Y by like 0.5%

Photo is from a first draft.

On the final version, I added an arch to the top of the exit slot so it would print cleaner without supports.