If the case needs to be waterproof then you will need some method to get the heat to the outside. You may be able to make the case of aluminum to act as a heatsink and mount the transistors to the case. That will make the case hot, of course.
If you want to connect more than one MOSFET to a single heatsink then you either need to use MOSFETs with isolated tabs, which some have, or use an insulating (mica) washer and non-conducting (nylon) screws to fasten them to the heatsink.
And, of course, if you attach more than one transistor to the heat sink then you have to add the power from all when calculating the heatsink temperature rise.