I would suggest contacting Geoff Miller, current owner of Rigby/ Rouge River Arms. In an article he did a few years ago for Double Gun Journal, there was just such a standing rest which he used to regulate rifles. This was standing rest, adjustable for height, and it had wheels which allowed it to be moved around. I know this rest works because I shot a couple of doubles from it during a visit there in 1999.

Geronimo