not a bad job on the actual construction.
but like the others said. that is not the correct port width for the start of the vent per chamber.
this is what i would do.
the port is 13" high. so on each side take 4 blocks of 3/4" mdf PER SIDE. that are 6.5" tall by the width of the pieces already there then glue and screw. that would make the two port beginings the correct width.
next time it should look like this.