I have an 855 with hoe - so behind the seat is not an option.
I fabbed a angle iron bracket and used 2 square U clamps to fasten it to the ROPs on the left side, about shoulder high. I have a 24" tool box mounted to the bracket, centered on the ROP's tube. After a couple of bangs with the elbow / shoulder I know it's there and haven't hit it anymore.
It sticks out about the same width as the wheels, so doesn't hit stuff unless I'm on slanted ground, or things are growing crooked. Got the normal stuff in there - pins, tools, etc. in it.
I was going to put it on the back of the tube instead, but ended up putting lights back there instead, so it sits on the front. More accessible there anyway.