I used teloscoping monopods that I bought from walmart for around $7 a piece (in sporting goods). Just screw the gun rest part off and screw on a piece of brass tubing with a bearing epoxied on the end. They will go as hi or as low as you could want, light weight, and colapsable for easy transport. If you ever need a monopod for shooting you can always screw the gunrest back on. The ball bearing makes them nice and smooth operating.
PS. I made a spike for the first one but then found that it works just as well to cut the end and smashing it in a vise. They are alluminum and plenty sharp for pushing into the ground without filling with dirt.