I assume you have read the VRM builder's guide.
https://www.trinityamps.com/docs/VRMBuildersGuide3.pdfI have never installed a Trinity VRM, but according to the builder's guide, in a Tweed amp you mount the MOSFET on the side of the chassis, external to the VRM board, and run wires between the two. Also a couple of additional capacitors are needed at the amp's inputs.
Yes, steel is better than air for dissipating the heat that the MOSFET can generate. Don't forget to place the insulating pad between the MOSFET and the chassis. This is needed to electrically isolate the MOSFET from the grounded chassis because one of its terminals is also exposed at the back of the MOSFET.
The VRM circuit is a simple one, but it has caused problems for several kit builders. That's why it's better to have the amp working properly before you install anything that messes with the B+. That way if you run into a problem, you know it's just the VRM causing it and nothing to do with the rest of the amp.