Working on the Propex heater install.
One of the headscratchers is how to seal the underside. In the picture below the heater is sitting bottom-side up. The black pipe is the air intake and the flex pipe is the exhaust.

Those two pipes will be connected with pipe clamps and nestle in two 2" holes in the floor. The heater sits about 3/8" off the floor so that makes for a lotta room for water, dust, and mud to come into the galley.
The aluminum flashing is my fix. It'll be screwed under the floor and sealed with high-temp silicone and a couple of sheet metal screws.
Tony