Holding a frame securely without damaging the tubes is often a challenge in framebuilding. I make and sell these tubing blocks in a range of sizes (www.alexmeade.com/Tools.htm). The Raleigh Twenty has a seat tube diameter of 1.294", not a standard diameter. This block is made specially for holding the Raleigh Twenty frame.
Photo and caption by Alex Meade: http://www.alexmeade.com/