Ported upfiring box

[silerh7]

I have a 03 sierra 1500 ext cab and i'm planning on installing 2 alpine type s 10" in separate enclosures under my back seat. I don't remember the measurements exactly but it's something like 21 x 15 x 7. my sub wants at least .74 cu ft of airspace in a ported enclosure but performs best at 1.4 cu ft. Do you have any suggestions? I've also bought some poly-fill to staple to the walls

[Ak_grown]

are you looking to build your own box? a prefab box works as well, just check the seal where the walls come together add silicon if needed. i prefer down firing myself.