Size M should fit most of spools.
Lid: slide it from side.... use some tool to get it over first counter. Then is should be easy to slide all the way.
To remove lid use small stick (2mm allen key) in the hole.
I printed lids without top/bottom layers, only hexagonal infill + perimeters.
Printed from PET or PETG. Then I can dry it in food-dryer on 70°C (together with filaments).