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.
I built one using 2" square tubing that goes in my receiver hitch on my ATV or my truck
then I built a U shape out of 2" angle iron and welded that to the sq tubing then drilled a 1" hole in both angle iron risers and put a rod thru it for the wire have used it for about 4 yrs works great
I have a neighbor that has a wire roller for his little Nissan truck. He bought an extra wheel and when he's ready to roll up wire, installs the wheel on the rear, jacks up that side of the truck, and puts it in gear. The truck rolls it up right on the wheel...
I once took a 3" length of 1 1/4" threaded pipe, placed a threaded end into the ATV hitch (with large washer on top with nut over that and another large washer resting on that, larger washer under hitch and then a followup nut) and then took a 1' diameter -1" thick wooden disc running it down over the pipe (drilled a 1 1/4 hole through the center) to rest on top of the large washer. Then simply lower the wire roll straight down over the pipe to rest on the wood disc which will rotate with the wire roll. Attach one end of the wire to a post and then drive!! If the stress on the pipe seems to be too great, just tie off the top of that verticle pipe to the rack on the back of the ATV to stabilize it (a bungee cord will work just fine). Works pretty good!!