Heh. I actually thought using the beetle shell as a container, but this is even better.

Yeah, it's still technically a container for the flying device!

Jokes aside, the other guy idea of a magic apartment inside the shell is tickling my mind.

It would be a bit like the castle gangster from One Piece, I assume?
He could turn his body into a castle, which people could then enter, thought they'd be miniaturised in the proccess. They'd then return to their proper size once they left. In one episode, they used the effect to fire full-size cannonballs out of the character.

Or, would the shell just serve as a portal into an apartment? (Which would be similar to an idea I once had about kangaroos who shared a "pouch space", where any of them could put things in and pull then out.)

I play Pathfinder (tabletop and PC games) and in the game's logic there is the possibility to have a magical closed space easily accessible from a weightless bag-kind item. I'm more inclined to see Kimera's shell as a portal more than a house-doll with shrunk furs in it. Like, you pop it and there are stairs going down inside. I'm doing a doodle, let's see how it turns out.

I admit they had a cool idea about shrinking cannon balls x3

So, kind of like the suitcase from the first Fantastic Beasts movie? Where the character set it on the floor, opened it, and it had a ladder going down inside.

That's good delivery ^^