using a buggy chassis as a heatsink

[othello]

This is absolutely not a stupid idea.

Yes it would be effective. It's the cheapest, biggest and "lightest" heatsink which is also used by nitro powered cars. You can tell how good it works when you feel it warming up ... measure the difference in temperature between motor and chassis plate. This will tell you how effective it works. The lesser the difference the better the heat transfer. The motor mount has to be designed accordingly to ensure a good (quick) heat transfer from the motor to the chassis plate. An additional clamp around the motor will improve heat transfer when mounted to the chassis. Chassis flex has to be adressed though.