Box modifications-B2 IS12 Help!!!

[VinteaRares]

Hello everyone. I have a box from a previous build, that is all ready carpeneted, and it looks ok and it has the right exterior dimmensions. Inside it has total 65liters(2.32cub ft)

But for actual sub i need to modify the port. Because i had a Golf IV the backseat is angled and i want that the sub box to ocupy as litle space. Actual port is designed to be paralel with the walls of enclousure and the board from left is has the shape of trapezoid and the back board of port is a rectangle.

Because the buttom and the top are not equal, it's twice; i had problems calculating the actual lenght of port because of trapezoid shape.

The question is, how can i calculate the lenght of port when the bottom is twice of the top and one side is angled??? By doing a aritmetical sum between top and bottom divided by 2? Because if i do so, i will have the port lenght needed(30cm- top is 23 and bottom is 40)

If the calculations fail(i hope don't), i have another option. To make port parralel with the board where is screwed the sub(on the front of the box), but in this case the opening of the port will be near the subwoofer cone and i think that the air flow and velocity will be destroyed. Ideeas? Experience?

I made a diagram where i hope you will understand

The opening for actual port is allready made on the face of the box where is placed the sub. I need to make a width-er hole because the actual port will need to be twice the actual opening.