Often some of the handiest ideas are so simple we all think—why didn’t I think of that! A while back I ordered a set of these “Quick Corners” from Woodcraft. http://www.woodcraft.com/product/124220/quick-corner-four-pack.aspx
I have found them very useful several times, In the past, like most of you I was looking all over the shop for something with the right radius to mark a rounded corner. Usually it was a can of putty, or maybe a paint can or anything round. These are so simple to use I wish I had had them years ago. As you can see, this set has convex and concave arcs in multiple sizes, and 45 degree angle in several sizes.
They all feature a built in lip so set-up is a breeze. Just hang it on the corner and mark.
Thanks for looking!