The only place your going to find an enclosure built to JL spec's is to buy one from JL.
Most pre-fabs enclosure's you find out on the net are built to generic volume's.
However, you can get around this by merely buying a pre-fab close to the volume on the other side of the jl spec'd volume, figure the volume difference and throw something inside the box of that volume displacement to bring it inline with the spec's.
Be very midnful of who your going to buy a pre-fab from because the assembly of the enclosure may or may not be what you think.
Some pre-fab enclosure's that are carpeted will have the carpet tucked in between the joining edges of the mdf - weak azz joint, leaks like bad case of the clap. This gap hidden by chaulking the inside of the enclosure joints so all you see is the chaulk, not the gap in the joint.
Sure the gap is chaulked - ergo - sealing the interior, but, this does nothing for the lousy assembly of the enclosure.
This is why people hate pre-fabs, sure, their cheap, cheap materials, cheap labor, cheap quality = Cheap for you that you want to buy it.
IMO, Go to Home Depot. Buy a slab of 3/4" MDF for 16.00. Have them cut the wood to the specified dimensions (for free). Buy about 3-5 containers of liquid nails (4.00 each). Screw the pieces together. Seal them with the LN. Save a heap of money, and build a far superior box than you would otherwise be buying.