This is what I did in my Duster. The material is .033" aluminum. You need to seal the edges somehow. I'd use some sort of adhesive-backed rubber strip or something. Don't forget to make access holes if there is wiring that needs to run through it. I don't have a pic of it but there is another sheet on top of the package tray covering the speaker holes and defrost vent. I used 3M two-sided molding tape to secure the package try panel, that stuff is really strong.
It took some effort to get the panel shaped around the wheel wells. If you have wheel tubs it would be much easier. I made a large cardboard template first then laid it over the aluminum sheet, drew the outline and cut out the shape with snips.
This image does not show it but the panels are screwed to the structures now. You could rivet the material in some places but there are a lot of gaps that would be too wide to do it everywhere. Sheet metal screws get the job done.
Eventually the rear firewall will be covered by a finish panel. I made the little platform at the bottom mostly so it does not look like a gutted race car but also to hide/protect the wiring harnesses coming from the trunk. The tops of the platform are removable in case I need to access the wiring. I can also put stuff on top if need be, it's solid. Most guys would leave that area bare or maybe put carpet over it but I wanted something that gave it a more finished appearance.
Duster rear firewall.
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I actually got the idea for the rear firewall from Gary Ball Interiors. He does a lot of Super Stock recreations and is top notch interior guy. Here's some pictures that he put on his FB page.
Credit - Gary Ball Interiors
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Finish panel. Credit - Gary Ball Interiors.
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