T-Forcefield is the highest rated back protector out there currently. It has no hard or metal, aluminum, etc. made into it that will injur you just as much as the fall. I did alot research on this before I bought one last year and I have also tested unfortunatey, two thumbs up.
But to also your question Kev, you shouldn't have a problem with sizing, like someone stated above, just remove the foam that is there and carry on.