Hi rb, I used Ross's idea on my boat - to glue in blocks for my footpedals. Another idea is to cut "U" clamps out of plywood. Let one leg of the U hang over the workpiece and the other leg hang outside the hull. Use wooden wedges in the slot to get the right clamping pressure.

I use this method fairly often and it can work well. Hot melt glue will come in handy to keep things positioned while you are wrestling the clamps.