I would not use an ap mat in a door for a few reasons. For one moisture will ruin the membrane material. Not too sure why you a stuck on low qts for midbass. Higher qts is better suited to achieve a good midbass response. Plus ib maybe your best option. Plexi is not the optimum choice for enclosure materials. Especially the thin stuff you would be using. Hard plastic cutting board material would be better. Although like I stated ib is probably best for you. IMO for your install a two way would be best. Can you fabricate a small full range driver in your sail panel, dash or a-pillar?