Some years ago, whilst visiting Pro Machine Tools, at Barnack, I saw what I call an "Infinite Vice" on an Austrian Milling machine.
After returning home, I made one using three pieces of steel, about 1" square (25mm for metricians)
Two pieces had side cheeks (1/4" or 6mm plate) attached to make a snug fit on the sides of the table, and keep things square, drilled and counterbored to take two M8 capscrews going into Tee nuts, to clamp down. One of these, the moving jaw, had a centrally placed dimple made with a small centre drill.
The third piece had the Tee nut facility, but carried, centrally, a 1/2" UNF setscrew with a small ball bearing in the end, to act as the pressure screw. (Any fine thread will suffice. Metric coarse threads are not ideal for use as pressure screws)
In use, the "plain" one was secured to one end of the table, with the work against it, and the second dimpled one brought up to it, but not clamped down until the third piece had been clamped down and the pressure screw tightened onto the moving jaw. Finally, the the second, moving jaw, was clamped using the M8 capscrews and tee nuts.
It did not take long to make, and although not often used, is invaluable when needed.
Howard