Tortles should be allowed to wear rigid external armor pieces: gloves, hats, and boots. Wearing any will either prevent them from retreating into their shell, or will fall off if they try to. (Possibly both - if the ordinary behavior is for them to fall off, a cursed piece of armor would presumably prevent them from retreating into the shell.)

Tortles have a Dex-based chance of avoiding getting decapitated by ducking their head back into their shell.

Bones of tortle players contain a tortle shell, which is useless except as an indicator the dead player was a tortle, or as polyfodder.

Tortles who have cold resistance don’t stumble when walking on ice, because their feet have claws to give them a good grip.

Tortles can wear hard helmets and gloves and a cloak, but as long as they wear any of these they are unable to hide in their shell.