Can't help with 2 x 12" subbies but the link below shows my custom box for 1 x 12" sub.
My criteria was to have as much floor space as poss and also to be able to utilise the smaller portion of the folding seat.
2 x 12" subs would require around twice the width but I fear you'd lose the use of the folding seat.
Cost was around $70 ($20 for mdf, screws etc + $50 for upholsterer.
http://www.fordforums.com.au/showthread.php?t=101891