I would also echo Mic's comments. The material won't be analogous so the only thing you'll get out of it is validating the shape. Don't be afraid to commit and just use your final material. Couple of caveats or suggestions (from an amateur) though:
1 - Find a place which has rems available for cheap - I have a local place which deals with cut offs and scraps, but ends up having billet and slab along with extrusions and tube available at a reasonable price. It's easier to bin something if you didn't pay a kidney for it.
2 - Buy supplies ahead of time - I knew I eventually wanted to mill an A/C bracket. When I saw a big piece of alu billet at the place mentioned above, I bought it even though I didn't have the bracket in my head yet. When I find decent pieces there, I grab them knowing I'll get around to using them (to a point- I haven't been back there in a while, but I do still have some billet and slabs hanging out in the shop).
3 - Make a cobbled together piece with scraps of stuff - Gets you a tangible thing to take measurements of off and visualize. Like I did in this post:
https://www.gt40s.com/threads/rcr-40-31.20158/page-11#post-318793
4 - Use that cobbled bit to make a drawing - Even a basic drawing is fine but for me, using Sketchup helped me consider tool paths.
5 - Think ahead - It's like a chess match, you need to keep the end goal in mind and sequence your cut paths to always have a square datum to reference. Takes a little while to think about t, but evidence is directing me to think that you're a pretty smart guy.
Based on your prior efforts, you'll do fine with this. I'm no master but feel free to ask questions off line if you don't want any more thread drift from this lovely D-Type build thread.