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use the simple technique to transfer the shape of ur trunk to cardboard. cut out the cardboard to the shape and transfer the cardboard to wood once the shape is perfect. this is how the pros do it.

 

Using the cardboard is the simplest, most direct way. You eyeball the shape and then start trimming. Have a good, sharp pair of scissors though. Nothing more frustrating when you are trying to trim 1/8" or less of cardboard and it just folds under when you try to cut it.

I've also tried the much longer way of marking out distances at 1" intervals back to a common parallel point. Sounds kinda confusing but if yer hung up on accuracy, it can work well. Dukk showed me how to do this on my Mustang install back in the mid-90's. You can see the lines on the wood going out to the curves, spaced equally.

 

^ lol, I still use that method sometimes depending on what I'm doing. Really I find that or scribing about the same speed wise. Just grabbing a chunk of cardboard and a razor knife can work out well too with a little practice.

For the OP - if you were asking more how to mock out a 3dimensional shape for the panel, IMO the quickest way to get good results with that is to just grab the cardboard, knife, and duct tape and go for it.