This is a case for a CMMG .22 conversion kit that will also hold a standard BCG. Now you've got a safe, clean place to stash your BCG when you want to use the .22 conversion. The rubber cap for the conversion will fit inside the BCG.
it's got pockets for 3mm x Ø11mm rare earth magnets you can get from Amazon (or wherever else if you don't want to make alimony payments for Jeff Bezos), and I just glued them in place. Gel CA glue works fine as does 2 part epoxy, but it's messier. Be REALLY sure of your magnets' polarity before you glue them in place or you get to print it twice. Not that I would know...
It should hold the lid on strong enough that you can shake it with a BCG in it and it shouldn't open.
Print it on end so you don't have to dig a bunch of support out of the inside. Or don't. Whatever blows your skirt up.