I used cheapo vise-grips with leather pads (see Shop Jigs & Fixtures page) to hold the scales down. Sometimes you need to flex the wood in certain places to make the tiny gaps go away.
Of extra importance is the Acraglas fill on the spine where the filework is. We are trying for no gaps, no air pockets, bubbles or anything unsightly. The filework is an eye catcher and you don't want to have a big air bubble hole in the epoxy there.