My best suggestion is to build a small table to simulate the lower part of the cab, and then mock everything up in cardboard.
I actually built a full rolling table out of 2x4's and plywood, to use as the base of my cab.
Once that was done, I mocked up the sides and top to the rough size/shape I wanted, and then staplegunned these onto the base and taped them all together until I got the dimensions and shape I wanted.
Once I had those dimensions, I cut the plywood for the sides, and screwed them to the rolling table, and then did all the fill-in pieces.