Here is the construction detail. I purchased three pipe connectors from a local hardware store - these consist of the rubber and the two clamps each. I then bent a piece of steel in a vise into the "T"
shape that you see here, and drilled a hole into it for the shelf. It was then placed under the pipe clamps before they were tightened. I'm not sure if the rubber dampens any vibrations, but at least it
prevents scratching the legs of the tripod.
The shelf itself consists of three 3/4" x 3/4" angle irons with a hole drilled into each end for a bolt. A piece of plywood was then cut to fit between them.
The only thing to be careful of is to rotate each of the "T" metal supports so that the tripod will still fold up. If they are all rotated the same amount and direction, it will still fold up normally.