The first thing I did was to measure the dimensions, then cut a cardboard template to fit the opening. Since I had already built a grille using similar materials and methods I was confident in the process.
The template is your guide, if it fits in the space and you build the panel to fit the template there should be no problem. However if your car is like mine and has been banged up and not repaired exactly correctly there will need to be an "adjustment" factor. My rear panel was kind of wavy, the filler neck was a little tweaked, and the bumper was bent a little. Come to think of it the tail light assemblies were a little wonky too. Don't let this faze you, just build it as if your car just came off the showroom floor. I did establish a center line for the template to locate the filler opening. I then made another small template to position the trunk lock opening. I should have made a preliminary light paper template, then transferred both of the locations to the cardboard. I just introduced more opportunities for mis- alignment. I also wanted to use some scrap aluminum sheet for the center of the panel. If you are careful to square up all your measurements and cuts I guess it would be okay, I just wanted to be in the "re-purposing" groove. I used the sliding shade panel from a 70's Buick sunroof I had laying around. I had used this for part of the grille too. It just makes it harder to make accurate measurements, since you don't have a "factory" straight side to use as reference. Still it can be done successfully.
I mocked up the panel by placing the mesh panels over the center panel. I still needed to cut out the opening for the filler neck and trunk lock. I had a small hole saw, approx. one inch, from a wooden door lock mounting kit. I drilled the small hole for he trunk lock easily. I used the saw to drill a center hole for the filler neck opening which I enlarged with tin snips. Not a real good idea. I couldn't make an accurate circle and tried to grind it smooth with a rotary stone. After some frustration I went and bought a 2 1/4 hole saw at OSH and made a new center panel. This came out much more satisfactorily.
Now I had two mesh panels, a center panel and I had to build a frame to hold them. I was going to build a frame work out of 3/8 in. flat aluminum stock. Two long strips on the top and bottom and two shorter ones on the ends. I had initially used screws to hold the grille frame together as I mocked it up, but this time I felt it would be easier and quicker to use some short aluminum rivets to put the frame together. I assembled these directly on my template. I placed a wooden panel under the template so I wouldn't drill into my table. I used just one rivet at each connecting point. This frame was going to be covered by the mesh panels and another set of aluminum strips. The mesh is sandwiched between the inner and outer panel.