Check around for a plastics distributor local to you.One that sells Lexan sheet and the like. LOCAL because shipping is expensive, especially a whole sheet.
If they don't have STARBOARD brand they will have HDPE sheet which is the same.
Years ago I got a 54" x 96" x 1/2" sheet for $245 from Emco Plastics in Cedar Grove, NJ.
I gave them a cut layout and for $20 they made 5 cuts on their machine. WAY better than dealing with a full sheet yourself.
All the pieces fit in my Ford Explorer.
Also, for that application (the bulkhead not the trim & door), consider using the cheaper and lighter vinyl board like AZEK or the cheaper knock offs that you can get at Home Depot.