I've been face with a similar problem for the nose of my Aerobee 150 booster. There's a company I've used at work, Compotool, that makes a ceramic based tooling board, https://www.compotool.com/ct850-tooling-board , that can take high heat and machines rather well. Like many ceramic based materials it's a little brittle on sharp edges and undercuts but it does work nicely. I've personally tested it to 775deg.F for 1 hour with no distortion or cracking. I'm going to give it a shot on the cone of the booster (right under the nozzle of the sustainer) . Might work for your needs. FWIW