18" RD Heavyweight v.2

[RAM_Designs 5,000+ posts]

That's assuming 3/4" MDF.

 

[spl_robb 10+ year member]

I don't see how that first port piece is going to be 16". Depending on how the pieces overlap(which you can't tell), you'll have three possibilities.
1) 15.75" if it's sandwiched between the front piece and other port wall.

2) 16.5" if it's overlapped by the front and overlapping the other port wall(or vice versa...overlapped by one of the two other walls).

3) 17.25" if it's overlapping both the front and other port wall.

did'n know that had to be that specific, but yes, its sandwitched with the front piece so it has to be 16 final including the 1.5" of the curb and front piece 3/4 mdf