I saw this being done at the local range on Sunday after our BGR shoot. One of the guys checking his sighting in.

I don't have a pic however will try and describe it. At the non bench rest end of the range they have a section where there are steel circular columns cemented into the ground. These are approx 1.5m apart about 2m high and each one has an adjustable tray to put your ammo in. So you can shoot prone sitting kneeling standing etc.

The next bit was obviously purpose built by someone and it was a piece of solid ply designed to fit in between the posts and on top of the trays on either side. Quite stable. The ply bench was probably about 1.5m wide and 1m deep With the adjustable trays you can get any height. It had a semi circle cut out of it and that is where you stood. The shooter had some some solid foam padding for support and it all looked stable as I said.

Wish I had a pic however you get the idea.

Regards
Rick