What a good idea – why didn't I think of that!

On Chinese 6×4 types, it's not difficult to take the back plate off and fit it to the front jaw:

For holding short items, I made a clamp. Can't find the stupid thing for a photo, but it's just a flat plate about 10mm thick about as long as the width of the fixed jaw and 60mm wide, with a couple of studs to hold a bar that clamps down on the work. Crude, but quite useful.
I do like Gary's solution though – it probably grips better and with far less fuss.
Dave