I'm designing a mosfet PWM device with a rather high current. According to my LTspice simulation the mosfet has an average power dissipation of 20W. The mosfet had a TO-263S package ( https://fscdn.rohm.com/en/products/databook/datasheet/discrete/transistor/mosfet/r6020pnjfratl-e.pdf ). What would be the best setup/design to get rid of this heat? I have two of those mosfets next to each other, so the heat sink should be able to dissipate 40W. Can i just connect the heat sink to the top of the case or are there better alternatives?

EDIT: Datasheet talks about a maw PD of 305W, how can this ever be dissipated out of such a package? Are there special tricks to do so?