Thanks for posting this Chris. I have been messing around with a half size caul and ones that I have knocked together to either reflex or straighten a stave, and have ended up having to heat an area more than once, which has led to to weak spots and some broken bows. I am going to build me one like yours that will do it all in one heating.
I have a quick question about your heat gun. It looks just like the wagner that I just bought. How far do you typically keep the gun away from the surface you are heating and do you use the high or low setting? I've only used mine once so far and it looks like 25 bucks well spent.