For trunk sub the only two applications that work are corner-loading or infinite baffle. The latter is cheaper. Look at what TMR and Kmarei did. If I didn't already have the custom enclosure I would go IB with a single 12.
Mobridge DA2 > JBL MS-8 > JL Audio HD600/4 > (3) Hybrid Audio L3SE & (2) Rainbow Cal25; JL Audio HD900/5 > (2) Morel HCW-10 & (1) AE SBP15