Now that's a good idea Landrover and I'm going to use it, thank you.
Update
I got one of these bales today, farmer loaded it into my trailer and I towed it into the field, left it in the trailer, used 4" nails to pin the targets to the bale and off I went, took six shots to zero at 35yds then moved back to 70 yds and zeroed again.
Such a good idea Landrover , thanks again mate, it's saved me no end of work. Have one on me