I recommend using 6 x 2 inch straight timber. Fix it with M12 or 1/2″ carriage bolts. Rounding the feet gives a more solid vise. The chain should be above your instep else you will trip over it. A wooden tool tray should be fitted with wing nuts on at least 10 mm Ø bolts as that increases rigidity. Do round the top or at least saw off the corners. Rounding allows you to grip saws longer than your vise. Spring-clips at BOTH ends of the chain allow quicker adjustments of height than a single clip.