Edited: Just to be clear, are you talking carrying fuel on the RZR or on a trailer/in a truck bed?
I’ve seen aluminum fuel cells in the 20+ gallon range for $2,000+. I have a 5 gal IMS auxiliary fuel tank in my 1000. I have 14.5 useable gallons of fuel. On extra long trips I carry an additional 5 gallon jug.
I always liked the 5 gallon metal cans just cause they are easier to lift. And the metal ones won’t let the gas vaporize over long periods of storage. I have also seen big packs of gas in a bag type unit that looks like a miniature waterbed. 20 gallons I think, but I can’t imagine how that is trying to pour it.
I use 2 of the Flo n' Go Duramax gas caddys 14 gallons each I strap them down in back of my pickup when I go for week long trips works great you stand them up on there end open the vent open the the shutoff hold the hose lower than the tank squeeze the handle and hold and you will filler up in no time. And I have six less 5 gallon cans in my truck.