I'm sure you've already thought of this, but instead of pouring something inside the door that will eventually settle and pack down, how about cutting into the door,
taking the window and lockset out and applying reflectix that way? I know a number of Burro owners have had reasonable
results by jamming reflectix between the hulls using every small entry point they could find.
You could then fiberglass/gel coat the repair. If you don't like the final result, make it obvious by adding a magazine rack, shoe rack or garbage pale. OR, coat the surface with a DIY solution like Grizzly Grip
(it doesn't need to be "rough.")