Call up Light Monkey (or probably whatever canister light manufacturer is popular in your neck of the woods) and ask them for an undrilled canister. Not cheap, but will definitely be the best depth-rated option. I have a LM can for mine and it's great. Holds a PLB, a chem light, a dye pack, and has enough room for something like a protein bar or similar.
If you remove the bottom screw clamp and use a tight loop of bungee instead, you can easily take it off without having to unthread it from your harness. Use a small d-ring attached on the top screw clamp and you can use a stage bottle leash like a wrist strap so you can manipulate it better in rough seas and don't risk losing it.
Wookie over on SB had a bunch made up like this that he'd issue to divers on his tech trips. The PLB's were expired so he gave a few away when he sold the Spree. He may have a couple left, but the thread should have some pictures to give you an idea at least.