Take a trip down to Pepe marine on Hayling Island, they fabricate the range of Aluminium fishing boats in the shed alongside. Its an amazing process to see, Ali hulls being welded up from sheets over a former then lifted off.
When I was rebuilding transom on my boat they supplied and cut to shape a sheet of Aluminium 15mm thick and about 120 x 75cm for a very reasonable price. It was an off-cut from one of their transoms.
They have lots of off-cuts, are very helpful, and take a genuine interest what you are doing.
Oh, you are probably making your swoopy not an 'A' Frame piece of sculpture for the project.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club