Volume of fiberglass enclosure

[heebs 09gts]

I know there is multiple sites with box calculators out there. But I would like to build a fiberglass enclosure. I know that it needs to be a 1.52 cu/ft sealed enclosure. I know I can use foam pellets to check the size of the box once I'm done. Other than that I'm kinda lost!
I want it to fit in the back corner of my trunk, so its going to be a weird shape.
How would I start to make a box, accurately to the size I need? Other than just guessing?

[heebs 09gts]

I found this nice write up that I found to be quite helpful, gives a nice walk through of how its done.

AJ Quick - Random Text