I would ditch the second 10 idea, up that 10 under the dash to a beefy 12 and call it a day. Plenty of room under there for a proper 12inch box .
Current: 2008 Super Air 210TE
Previous : 2000 X-Star, 3000lbs ballast, LTR 330, Acme 1235 14.5X14.25prop. 1200 hrs of flawless fun