One thought. I wonder if the (loose) heat sink is really necessary. I suspect the heat sink allows a continuous rather than intermittant use of the controller. I wonder if a fifteen second run (stop to stop) with mostly momentary inputs would generate enough heat to be a concern. I had a commercial trim motor controller that was about the size of two nine volt batteries. I had to think of why one was so robust and the other so small. The intermittant trim use of the little and the continuous design of the big one one was the only thing I could come up with.
Lowell