While I was waiting for .... inspiration, I guess, I made a radius block out of some one inch thick acrylic that was left over from a sign job many years ago. (More of those perks from the printing industry). Also from the job was a piece of steel pipe that was 28" in diameter. That really doesn't have anything to do with printing...but my boss decided we needed a huge smoker/bbq pit. We are in Texas...and he came across some pipe, and...well, I should probably stop there. It did get finished...ultimately...one way or another. Anyway that pipe made a nice 14" radius, so I smoothed a section of it and sprayed some adhesive on it and stuck some sandpaper to it and with a little bit of elbow grease ended up with a nice radius block.
I added some cork sheeting to each side to slow down the development of corns.
The flat side is perfectly flat and will good for making sure the fretboard is perfectly flat and straight as well as radiused.
SR