Poly will help reduce "peakiness".If the box is too peaky and you can't adjust it with settings, take the poly out. Poly changes the tuning of the box, and not always for the better.
Awesome write up, because I've used them before I sourced the connector and wires from e-acca for a total of $17.33 USD delivered, so if you're not in a hurry I'd recommend it if you want to save a few bucks.
3/4" mdf is cheap and heavy - I hate it. Quality 1/2" plywood (not the crap from Lowes) is much nicer to work with, and an enclosure can be made that's substantially lighter. You can make up for the reduced thickness with internal bracing.
Think about sonotube extending down into the spare tire well for added volume. It's just large diameter cardboard tube used for concrete foundation forms. The round shape, however, gives it adequate stiffness.
Sonotube + 1/2" plywood probably wont be as stiff as a 3/4" MDF, but it would be a fair compromise to get the weight down and would work fine for a reasonably sane subwoofer stuffed in the spare tire area.