Here's a sneak peak of something I've been working on:
finished model to show for it so far (in fact I have a second model with a very similar, but slightly more advanced, base that's waiting to be painted, but I... kinda lost interest). In this case, thanks to the larger size and depth of this 50mm base, it was much easier to install the battery and switch without having to make an entire custom base. Hopefully it won't be too long before I have a finished and painted model to show off.
While I wait for some stuff to dry on this one, I started experimenting with installing a battery into a 30mm base. 30mm bases are not only smaller, but thinner, and this is the real problem: it's very hard to put a battery holder underneath without it sticking out. However, I found that if you cut a hole in the top and add another layer above that, there's just enough depth. This works quite well because the top surface in these bases is actually recessed, so putting another layer on top looks fine and doesn't actually increase the height. It will still be a bit of a tight squeeze inside depending on the particulars of the model, but so far it's looking good for this test piece.
Here's a shot of the 30mm base so far, I'm waiting on some LED's before I go any further: