Took two pieces of 1 1/2" (rough figure) angle iron about 2' long and bolted them to the back rack so about a foot was sticking out with enough room in between them for the roll of wire, also it's nice if they are just wide enough you can cram a 5 gal bucket down in them til it hits the shoulder below the handle and the rod in back. Drilled a hole in the end of the vertical surface and ran a 3/4" rod between them with a pin on each end. Put the roll in unwinding from the top and drive. Word of advice: Don't use Rangemaster wire, it wrapped up on mine and about threw me over the handlebars. Would take a picture but I have sold that particular 4wheeler, and lately I've just been shoving a piece of pipe between the arms of the hydrabed and using it.
Edited to add: If you make one like I described above, make sure the inner surfaces of the angle iron close to the barbed wire are flat, so as the wire rolls it's laying against a smooth surface.