12 man
Is the correct terminology not a chocolate fireguard ?
I think all three idioms can be used almost interchangeably, but I agree (with my newly acquired magnanimity on turning 50 today) that there are nuances that probably make yours the most correct!
Chocolate fireguard - designed to protect, but cannot
Chocolate teapot (or frying pan) - designed to fulfil a function, but cannot
Tits on a bull - intended for something, but totally useless