Reuse > Recycle! My collection of empty spool cores was out of hand, and existing models for “lids” to turn them into containers were fine but either used too much filament, took too long, or were more than my simple needs. I needed stackable caps, simple and quick to print with no supports or other special requirements.

These are friction fit, but I do use wood glue to glue the bottom onto the cardboard core. Plain liquid school glue should work but I don't have any to try. Depending on your spool core, you may need to scale the model to get the fit you like, my profiles work well on on my wide selection of various vintages of Bambulab cores and Sunlu PLA+v2 cores (the spools that will accept a Bambulab refill). Scale in X and Y only, leave Z alone as the Z height is used to set part of the top surface pattern.

I have uploaded several .3mf files (print profiles), there's one that has ‘ventilation' and another with a ‘label’ that you could add text to. I use glue stick to stick labels I make from paper directly on the cardboard but the ‘label’ version is there if you like things to look neater. Yep, that glue stick still has actual real life uses! The ventilation is implemented with modifiers, going to object mode and selecting the modifier will let you change the sparse infill pattern and density to suit your fancy. Do note that the slicer may leave some stray ‘strings’ in the print, I don't know what's going on and couldn't fix it—maybe it's a bug.

You can also adjust the top surface pattern, I liked the look of Hilbert Curve but if you want to change it go to object mode and adjust the top surface pattern in the height range modifier.

I hope these are useful!