I build using floating dock pieces of blue foam. What you want? comes in 1' x 2' x 8' long. Cost? + $60.00 local lumber yards. in NJ.
Great fpr boats. The hull can / should be CAREFULLY hollowed out to save the long pieces for deck parts. I use caulk gun sizes of Silicone Rubber. Always really tape the used end carefully. Get the CLEAR only.

You wil DEFINATELY need plastic keels to stop the wind blowing the boat sideways all the time. Make the keels deep enough to protect the props & rudders. 2 are better.
Any size car radiator fan motor will work. Junk yard Make sure it runs on 2 brushes. Add a small used car battery from the junk yard & a ESC done.
Remember FOAM is super light. Put the big piece in the water to see how high it floats. 1' high piece lightned willsink 1" to 1.5"

Rich