I have the forcefield pro sub 4 back protector. Fits well under my suit and doesn't move around once its on. It claims to be the best in absorbing force and is reusable for multiple impacts.

http://www.webbikeworld.com/r3/back-...eld-pro-sub-4/

Most protectors have a sizing chart that you can use when looking for size. One thing I'd recommend is that if you're buying local, go to the store in your suit and try on the back protector. Back protectors can be different sizes/thicknesses and so you need to make sure you have room in your suit for it.

Otherwise I guess make sure it's at least CE lvl 2.