Try the DD box recommendations but, lengthen the port to ~22" instead of 16".~1500w is fine on them, but they can handle a good bit more.
I just did 2 2515's in a customers Suburban a few weeks ago. Did 8cuft w/ the 22" port ~16sq per cube. We ran that on a DD M2 and it was DAM nice. Alot stronger lows that you would think given the "on paper tuning".
We also have a pair of 2515's w/ CF on my co-workers truck.
Its a 4 door Ram w/ 2 DD M1a's strapped at 1ohm.
Now, I wouldn't recommend that kind of power on them for the average Joe...but if you REALLY now how to watch the subs its fine.