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.
My Arkel bar bag has a square of evazote loose under the removable floating liner. As an aside, it has a small hole in the middle of foam,with a small "half button-hole" in the underside of outer material for drainage.
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.