Several years ago I picked up three sanding "stones" with different size half round groves. They are about 100 grit and are 12" long square. They work real well for rounding leading edges and trailing edges prior to installing control serfaces like rudder, alerons, etc. Don't recall where they came from but over the years I never saw a tool I could not do without. However, the one Atlanta shows came from an old Goldburg kit, don't recall which one but included in parts cut outs. I made another out 1/8 ply at 90 degrees about 6" long with varying grit sand paper which works pretty well. Then again the razer plane is an excellent tool for any type of rounding block balsa.