In the rare event that I'm in a wetsuit instead of my drysuit I have a older Sealife Camera housing that I keep my keys in. Hasn't flooded yet, but I keep the keys in a ziplock style bag in that as a backup. I think I paid $15 for it and I keep a little lead in it to make it neutrally buoyant. GoPro cases work too, but depending on the key size they don't always fit (my Subaru key is too long), and although I've never flooded one I've heard of several people flooding theirs. Other things I've seen used: dead Sola light, PVC pipe capped off with a screw top and an o-ring, and those dive rated phone case housings.