I'll be doing a pair of 15's this year, maybe i'll have to try this way with the box. I have an 01 Blazer, with a pair of 10's the best placement was subs up and 2 4" external Aeroports towards the driverside, slightly angled towards the front. Did a 50 on the floor on 2K. Upgrading to 15's and 5K this season, was gonna try the same way, but maybe I'll have to try the ports back.
One of the competitors around here had a jeep with a pair of 15's, external 10" Octoport with subs up and port to the driver side and doing 55's on 4K. Slightly different vehicle, but roughly the same internal area as the Blazer.