Chimera makes various size (universal) 'speed rings' - new ones are expensive; $100+
I have a Chimera molded plastic universal type ring that fits a 6" dia. reflector. When I look at it, I'm sure I could make one out of hard wood without too much difficulty. They're simple clamps with 4 holes drilled at the corners, the hardest part to make would be the opening in the middle. There's plenty of air space between the reflector and the ring to avoid any heat damage, I think - it depends on how hot the modeling lights get - electronic flash, no problem.