I've used the Clover Hera marker from Amazon. It doesn't actually leave a mark, it makes a sharp crease. It might not be obvious enough for what you're doing, but they do have a skinny one for tighter corners. There is nothing to wash out, and it just makes a crease which you'll make when sewing anyway. The crease relaxes after a couple of days and goes away. To make it a darker crease, I put a towel under it & pressed harder.
This is after I tried blue tailor's chalk that wouldn't come out completely. Got most of it with a dry Magic Eraser, but it's still there, even after a wash. Never again.