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I made a free standing adjustable stand that serves the same perpuse as yours . It was not my idea got it from a book or magazine .
I have seen this idea before, it works well but the next generation is to use shevling hardware. The slotted standards and flat metal brackets are the best value for the money. Mount the standard(s) on your bench then just add the bracket where you need it, you can add blocks to pad the metal or "shim up" the hieght if you like. Multiple standards makes it easily customized to fit your stock, also the standard don't have a real top, this means you can flip a bracket over and use clamps to hold everything rock solid.
As an added bonus this system can be use as temporary shelfs while machining, assembling or finishig muliple parts.