Fixing the heater box, question on mounting motor

Flex-seal, RTV, whatever really. They all have some odor until the cure completely. If you covered the motor flange with some material that your gasket maker doesn't bond to ( Shipping tape? ) you could generate a good seal that easily separates again.
The factory used a dense foam as gasket material in a lot of similar places ( accelerator to firewall is one example ). I'm not sure what they used here. You might find a piece large enough in packaging materials,,, Whittle out a gasket.