Find yourself a 3" thick cardboard box or cookbook (wrap it in a plastic bag!) sized like the subs you're looking at, shove it under the seats and see how feasible it is. FWIW this is the thinnest one I found and it ain't great. https://www.amazon.com/Rockville-SS8P-Under-Seat-Powered-Subwoofer/dp/B079PRCP9W?th=1

Marine and powersports audio are going to have a lot of what you're looking for. This sees a lot of use in custom home speaker builds: https://www.amazon.com/Kinter-K3118-2-1-Bluetooth-Amplifier-Subwoofer/dp/B08SL6ZL6Q You'd need a clean power supply, maybe even go battery with some 18650s wired to charge when the car is off.