How about the pipe or shaft, across the top of the box of the pickup and two wooden stakes in the back stake pockets, (or rod fence post or what ever else is in the bed at the time that will fit the stake pockets, and put he roll or wire on the pipe or shaft),
I usually use our tamping rod and either two small fence post or two wooden stakes, yes a disk is help full if the wire roll slides to one side and starts to rub on the box, expecily if it is not a solid spool but a wire spool like in barbed wire, solid spools usually will not catch when turning. I find the spool horizontal seem to work better for me than vertically.
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If one would jsut weld a pipe on to the frame of the PHD and use that to put your roll of wire on I could see that working as well, that way you could unroll and still dig if necessary with out change over, If going down a straight line your first wire or one could put two holders on I guess and unroll and dig as one goes, drill a hole in the end of the pipe for a clip pin and slip a washer on before the pin, on the inside a small disk or steel plate would give a support area, at a light angle off of harizontal about 15 degrees there would be some drag and yet let it unroll smoothly,