Just wanted to point out that one big heatsink is very likely electrically conductive, and that you should therefore think about electrical isolation between it and all the transistor tabs.
There are thermally-conductive, electrically-insulating adhesive tapes, such as 3M 8810 among many others. They might not perform as well as the standard mica sheet with thermal paste and a proper screw attachment, but you gotta do what you gotta do.