2.3 c/f per sub AFTER displacement it´s perfect for a lot of 12" subs, after a few testings i can tell you that, for example, my crossfire BMF likes barely 1.75 c/f , what subs are you gonna use???
Anyways, about the distance, lots of people says that you need at least one of the measures of the port opening, for example, if you have a port 22" x 5" , then you need at least 5" from the hatch, for me this is only a myth, in A LOT of testings,i´ve found that my explorer likes the port no more than 4" away, we are talking about a 14.5 x 9 port by the way, while my swift the further the port is from the hatch the louder it is, my point is, that maybe generally speaking, you would be ok with at least 4" from hatch maybe?? I dunno
About double baffle, yes you can do it, but it´s more expensive,nothing like a good old threaded rod