All measuring and test fitting was done with the gas can in place.
I first measured the depth, leaving a little room for the Dri-Dek flooring that I plan to install. The only board that I actually 'ripped' was the bottom board on the front, the rest were left full width.
I made a basic framework with 4 short boards screwed together, and put it in place over the gas can.
I took a measurement across the back of the aft-most section, then played with the miter saw until I had the angle. Once the angle was right, I cut the board to length using the miter saw and paying attention to get the direction of the miter right. After that, it was measure, cut, attach, measure, cut, attach until it was done.
Some light sanding, and a heavy dose of spar varnish finished the project.
It is not attached to the boat in any way (except gravity) and removes in one piece. You can't stand on it, but it is surprisingly sturdy resting on the gas can. I got lucky enough with size that I can also carry 2 one pound propane cylinders under it to run my BBQ.
Edit: Thanks for the compliment.