I'm not sure if I can follow. You want a back cover which doesn't cover the sides and doesn't snap onto the iPad? Please tell me, if you find something like this 😉
Meanwhile, you could use a case where you slide your Pro into it, like Deane2000 said (maybe something like Kavaj leather cover) It's easy to get the iPad in and out of the case.
Or, you could just use the smart cover for the front and for the back side, use a film protector (just like the screen protector, but for the backside). You can find it on Amazon and I saw a few people opting for this, because they like the nakedness of their iPad.