ORIGINAL: frontier gander
Im sure that would work fine. However, the plastic rests usually are pretty light and a sand bag would help to keep things steady. I'd probably just buy a couple sand bags, and then fill it up with sand. I prefer sandbags over my plastic rest.
I bought some empty leather covered canvas shooting bags and filled them with sand,they work well. I shoot off thetailgate of my truck for a shooting rest, sitting on a chair. Cost was about $20 from Cabelas. I have one of the plastic jobs, but as Cayugad said it doesnt allow for you to hold your rifle, so I use that for cleaning or adjusting the rifle.
Chap Gleason