For guitars, some people use elastic substances- cords, tapes etc- hooked over screws on the sides of the mold, backwards and forwards over and over to cover the surface. Some people use dozens of individual spool clamps, while some use a curvable plate overlapping the top by a margin big enough to screw it down.
What I did here was more akin to the last example than to the others. If the top fits all the way around, without wayward bits that have gaps, then very little pressure is required to clamp it. You really only want the minimum of pressure needed to squeeze out excess glue because the top should want to sit exactly where you put it. I used 50mm bugle headed screws in a battery driver with the clutch set carefully to slip on very little pressure to make sure I didn't get too enthusiastic with the tool.
If I decide to make more guitars I might make a more formal arrangement than the separate pieces in the pic above. I may have to anyway for the back, which has much more fore and aft curve, particularly at the top of the long arch.