Most often, exterior plywood and sealed real well with some water sealing product, such as Spar Varnish. I'd do the edges a couple of times. What ever you do, do not use pressure treated wood.
I did mine from 2 layers of higher grade 1/2 ply wood. I got a 1/2 sheet 2'x8' from Menard's damaged board area cheap. I painted it with 5 coats of exterior primer and 5 coats of exterior paint tinted to mach the old paint. If not for the Oak 1"x6" I added to run bigger motors you would never think it was new.