Hmmm... Not to be a back-seat designer, but did you consider using a push-in/push-out SD socket? I'm not real sure on the mechanical dimensions, but it looks like you might have had just enough room for one, with a slot in the casing, to allow changing cards without removing the battery cover.

I'm still not sure that would be a good choice for other reasons, though; maybe too easy to pop a mounted card out.