iso box

right now i have two DX 12's....ive been thinking for awhile that i should have only bought 1 due to the space i have. so i have been thinking about building a iso-push pull type enclosure...what size should i make the box? .88 cu ft? and also how do i mount the external sub? do i just screw both of them together face to face, or do i need to make a insert so the surrounds dont touch?? or do u even recommend this box style, or is there another one u can suggest? i want a compact box that is loud! thanks!