I don't remember seeing a heatsink specifically made for a 20mm star, but there are some that have a completely flat top without a pedestal for an emitter.
You would then be able to mount a single star in the centre if thats what you wanted to do, or you can fit three 20mm stars in a mag head and then use smaller reflectors or optics.
I'm not sure who or where they were available from, but they are out there and a quick search will probably find them for you.