Not likely you have one (I do, and you're welcome to use it) but most any shop would. It would have the capacity to slice through the 6" piece on an angle or even a slight curve that might show up if it was scribed to the cabintop. You might know a shop with a bandsaw nearby that could cut it for you?

It would require fastening some sort of flat block to the round to ride on the table, otherwise bad things can happen, I know.

If it is just a flat slice, you could also do it with a handsaw, carefully, again clamped somehow. You can clean up any rough cuts with a sanding block and coarse paper.

Your heat choice makes sense to me. I too could never get ducts easily through my boat. Those heaters(Espar etc) are nice but use too much power to make sense with how I use the boat.