I Use A portion of a CELLULOSE Kitchen Sponge Wipe, They come in 6 to a pack, They are generally 8 to 10 mm thick and sized 16cms x 12cms from any supermarket/Kitchen drawer

Laying the object to be drilled onto the sponge near to the straight edge but not so close as to lose the cushion effect of the sponge, draw an outline in the sponge of the side nearest the VICE jaw of the objects edge,
Leaving a generous amount of excess sponge, cut out the shape and repeat for the opposite side.
Using a dab of "Bluetack or similar to hold the sponge in position along the edge of the object that will be clamped, place the object in the jaws of the VICE align it and tighten.
You now have the object supported along both sides and as you tighten the VICE the sponge fills the irregular shape with the straight edge of the outside of the sponge butting up against the walls of the VICE, And everything being held firmly in place and the sponge helping to avoid marking it.
It works for me.
Shaun.