Touch buttons and a USB socket are relatively easy to make, and I did them before. But a battery holder is a new thing, and it requires some thought. I made a number of sketches:

But eventually I decided to got with the simplest possible design that just might work:

Here is how it's supposed to work:

I'm not entirely sure how elastic the PCB will be, and how much I will need to bend it, so I assume some experimenting will be needed. I started with relatively short but thin tabs. I also included a footprint for a regular battery holder on top of it, so that if my first design doesn't work, I can still use the boards to test and prototype other parts.