Need help for new blow thru box for Crew Cab Denali Mojo5's 2-15's

[SKALADE]

Picked up a pair of Mojo M5's and a MJ-4200.1, so out with the old setup and time for a new box!!! Here's what I have... my cab wall is cut with an opening of 45" x 8". I want to build a slot ported, forward facing box. I had thought about building a down firing bottom slot ported box but can't some up with the design, dimensions, correct port volume.

Memphis sent me the specs for the internal airspace = 7.72net-9.889gr & port dimensions of 15.5" x 8" x 17.75"

My Dimensions are:

49" wide fits between wheel wells

17.5" - 18" tall

29" deep, that's the current box,

With the box needing 3-4" distance between the subs and the bed wall,,, Is that front area between the subs and cab opening acting like a 4th order? Isn't that area going to affect the port volume and determining the port size for the box?

So I am looking for suggestions on what to build to get the best result. ???

Chime in, will try to post some pic's of the old setup.