I've seen "replacement" cases offered on B&H and KEH's website. Maybe made by Optech? Although I sometimes use a half-case (original Canonet QL17 GIII) when wandering around with only the camera (to provide some protection against casual encounters with hard things like bus doors or building corners), my cameras mostly go "naked" inside a camera bag. I NEVER use the top half of the case. I think the ones I've seen are one-piece, made of a covered wetsuit-like material.
Regards!
Don