One of my favorite accessories is my netting that I put on the roll cage...under the roof. It allows me to put other clothing and supplies above my head and tight to the roof. It keeps things out of the way and dry, even when the rain is carried through the side by side with wind. In really cold weather the roof can sweat so garbage bags or dry bags help keep things dry as well.
I've got three gun boots and I have yet to find a good way to mount them to my side by side. I have had them mounted to my quads and am not a huge fan. I would rather keep my rifles in thick padded cases that I can bungie to the quad or in soft socks in a gun rack.
Obviously a winch is a necessity.