fist

I've been trying for years to get my hand in there without pulling every muscle in my arm and wrist and back. I could easily accommodate a fist or maybe even two but that reach is a killer

Use a rubber glove and a lot of lube and go very slow.

I've done this before and it was ok, not that great tho. If you want to do it my advice would be:
Go for a shit beforehand to clear the system!
Use a fuck load of lube
Take your time, its best to start one finger at a time to stretch the ring then after 4fingers just push the whole thing in
Have a towel ready
And relax and don't keep tensing your ring, you should feel a bit like you're trying to push a poo out
LUBE is very essential!! Buy some decent stuff