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I'd use a floating liner, only stitch it to the shell at the top edge, should be more than suitably waterproof without having to get into seam sealing and it saves you a lot of seams that need finishing.
If you find a spray adhesive that plays well with foam and fabric I'd be keen to know! Alternatively you can sew the foam panels onto the seam allowances, just the corners would be enough probably.
Thanks both - I'll start with 4mm foam and see how it goes.
I'm still trying to work out the construction and quite how the shell, foam and liner interact with each other. I hadn't thought about gluing the foam in place so that's another idea to look in to.