The face vise on my workbench didn't hold long boards rigidly enough until I added this adjustable "bench slave." And it only cost about $30.
First I milled a board to attach to the leg, making sure it was thick enough to fit flush with the front edge of the benchtop. Then I cut a dado right down the middle of the board, to house a piece of T-track (slotted aluminum track that accepts T-bolts; see Sources, below). When I installed the T-track, I left a gap at the top for installing and removing the support block.