I have two draft horses, and two Llamas that we keep on about 8 acres, split into two pastures. The animals are rotated between the pastures during the grazing season.
Here's how I fashioned my own drag:
I went to the local Lowes and bought a coil of chain link fence, six feet long. I had an old piece of 1 inch black pipe that I cut to about 7 feet or so.
I rolled out the fencing to 20 feet, and doubled it back on itself, so the section ended up 6 feet wide by 10 feet long, with two layers. I wired it together along the edges every foot or so with some cheap wire I had around.
I ran the 1 inch black pipe through the wrapped end of the fence, to creating a pulling bar. To the bar, on each end, I connected a 12 foot (or thereabouts) piece of small chain. This chain hooks over/through the tow bar on the tractor. I had to fashio a loop end the end of the tow chain to keep the fence from sliding back and forth around the tow pin as it went around corners.
I weighted the chain link fence with 6 or 8 cinder blocks (I forget which), each loosely tied down with short links of light dog-type chain. Also, the blocks are staggered across the fence to create a nice flat drag.
I've used the same piece of fence for 4 years now, and have not replaced it yet. Though it will stretch a bit over time, it's still very usable, and was inexpensive to fashion. Note that the diminsions here are NOT critical.
Its a bit crude, but for my purposes, it has worked just fine.
If anyone wants a picture, drop me a note.