BS170 is a good cheap MOSFET if you want to use that instead. It's also called 2N7000.
Another idea is to put the Pi on its side, and orient the heat sinks so that the air can flow from down to up in that orientation. (Orientation is impotant for E-profile heat sinks; not important for pin-profile heat sinks.)
If it's in a case, make sure there are plenty of holes in the bottom and top when it's in this orientation.
Heat convection may cause enough airflow that you get the cooling you need. That of course depends on the rest of the surroundings -- if it's behind a heavy velvet curtain inside a small drawer, nothing's going to cool it very well
I wonder what people using HATs do about cooling? I'm going to be running a small neural network on the Pi, and stick a HAT on it for controlling motors and stuff; I don't know how the chips will get cooled in this case, and the NN will use most of the cores with high utilization...