Foam arrived today. So I cut it down to size, then after I took this pic I decided to stick a bit on the front so that it filled the bottom of the case instead of having a deeper section by the neck:
Then stuck on some velvet-style fabric I once bought to use as a photographic backdrop for the guitars but found it was far too shiny. It doesn't match the case fabric, but who cares!
It's quite firm but still squashy enough foam. It's a bit tall and could be 10mm lower, but the guitar still sits in the case OK and there's no problem closing the lid:
I've removed the insert for the time being until the glue's fully dried. The polyester finish shouldn't be affected by the vapours (if the bomb goes off, the finish is so hard it should still be there, along with the cockroaches and Keith Richards), but better safe than sorry.