I've done quite a bit of wet felting in recent years, scarves, bags, hats, slippers, felt vessels, various containers for specific items, phone holder.. It's great fun - I tend to use bubble wrap and a towel for most things but have borrowed a cane type bamboo blind for larger items and things that need extra strength. I advertised on freecycle to try and get a couple for myself but no joy yet and my friend is happy to loan me hers when needed.
I keep trying to learn needle felting but am not sure I've the patience to do detailed work over a long period - may find out today at the friend's shops' 10th anniversary as they're doing demos and tasters. It would be good to have something simple and repetitive to do while watching TV rather than have to set up the kitchen table and stand over it.. as long as I don't end up stabbing myself with a needle of course!
Felt welly liners sounds great - I've done slippers but am assuming you mean long leg like knee socks or even over the wellietop and turned over? Details would be good FiB please