its a custom box that he can build and ship to you. Majority of the pre-built boxes online are absolute complete garbage. Custom enclosure is the way to go if you want great sound.
If 15h X 37w x 35d is the trunk space you are willing to give up, then you should do a single 18" or the smaller subs in a T-line box because that shallow mount pioneer 10 ain't gonna do sh*t with you big car. Plus pioneer subwoofers are near bottom of the barrel on the market.
Box is what makes or breaks your whole subwoofer setup, always go custom box or else you'll be disappointed with those cheap Chinese built pre-mades.