Have you seen the ES-52? Not a lot of physical protection there, it's incredibly shallow. Also, it's a thread-mounting hood, meaning any force applied to the hood won't be transmitted to the barrel as with a bayonet-mount, but rather to the front-focusing element with the STM motor connected to it.

Also, the thread mount means it cannot be reversed, although since the hood also has a front thread for the lens cap, that might not be a big deal.

I am compulsive about both hoods and UV filters, but I think I'll forego both for the 40/2.8.