I have a few filters for my 12-40 (62mm Ø) without threads. This means that I can't put the normal lens cap on when I've got a filter in place. I've designed this filter cap to go over the lens bayonet mounts, and over any filters attached that are less than 7mm in thickness.

I'd print it out of PLAFlex (what I chose), or PETG. It uses a small lug to lock into place, so you'll want a fairly flexible plastic. I wouldn't use PLA as it's fairly stiff, and has a fairly high surface hardness, and may wear down the camera bayonet lugs.