Lets hear about what you've already built

[Brother Rockeye]

I think this would suit your purpose for the range as you could bolt it to the benches.
it's a variation of one my uncle built.
basically it's a tripod mounted adjustable rest.he took a 1 inch nut,drilled the threads out, and welded legs to it to form the tripod(if you used 1"x1/4" flat iron for the legs the could be drilled to mount solid to the bench.).another 1 inch nut with 3"x1/4" cold roll arms sits above it for adjusting.thru them both goes a 1"x4" bolt that the rest is welded to.he used a piece of flat iron and an elk hide bag that he made,but for your purpose a piece of angle iron would do if it was padded.
any descent paint would keep it from rusting and the occasional squirt on the threads with a wd-40 or such would work there.stainless is another option if you're a better welder than me.
just a thought-if you bolted 1 leg down and left a 1/4" play you could still rotate the rest on the bench making it more user friendly for south paw shooters etc. and still keep it from disappearing.forgot to add that a washer welded to the bottom of the main bolt will keep it from being removed from the rest of the rest.

sorry for the crummy illustration-but you get the idea I hope.