I sometimes have long or wide boards that I need to clamp at my workbench. The face vise will hold one end, but the opposite end needs support. So I built this board jack to support the end opposite the vise.
To build the jack, I routed a narrow slot in the center of a board and then drilled a hole at one end to attach a dowel. The dowel slips into a hole in the front edge of your workbench. An adjustable support is fixed to the slotted board to hold one end of the workpiece.
I drilled a hole in the face of the shelf and inserted a carriage bolt through the hole and into the slot. Then added a knob that I threaded over the carriage bolt. When I tighten the knob, it keeps the shelf at the proper height to support the board, while the other end is in the vise.