When building a new guitar, it's convenient to have a convex radius block that matches the radius of the acoustic guitar you're building
 
Instead of placing sandpaper onto the top of the guitar and then sand the bridge on top of the guitar's soundboard, this convex radius dish will help preventing you from damaging the guitar's soundboard that you've worked so hard on already. 
 
Clamp this onto a vacuum hold down clamp, or use traditional "C" clamps to hold this convex radius dish down securely while you shape the bottom of your acoustic guitar bridge. 
 
It's slim profile and small 10"x10" footprint also makes it easy to store away when not in use. 
 
Comes in 100 grit sandpaper. We recommend 100 grit because you want a smoother surface before gluing the bridge onto the soundboard, and it's course enough that the small amount of sanding surface for the guitar bridge can be worked relatively fast without marking the bottom too roughly, and allows for touching up with the slightest effort for those who want an even smoother surface.