I found aluminum in 4x10 sheets at a metals recycling center today. They come painted tan. not the color of choice but will look good.
The price was very low, by weight it came to 44 bones a sheet. I think I'll still use the lexan for the build. However, the lexan will be 40x60" and I'll keep a spare $22 48x60 sheet of alum just in case I change my mind.
By the way, the metals company in San Jose California is Simms on Monterey rd.
Disclaimer, I have no affiliation to this or any supplier I have used.
Dave