How to bond PCB and metal for high power RF boards
From first principles:
Just glue the board to the copper sheet with regular epoxy. As long as you respect the temperature limits of the glue, and the bottom of the board is clean and flat. Keep the glue line as thin as possible by applying the minimum and then compressing it in a press to squeeze most of it out before it sets.
Reasoning: 0.1 mm of epoxy will have about 5 x less thermal resistance than the 0.5 mm of fibreglass in your board, so you don't need to optimise much more.
It won't be electrically connected to the copper so you might also need some screws near the RF connectors.