One of the recommended places for replacement boxes is
http://www.ssicustomplastics.com/
I dont believe they make one thats and exact size so you would have to open your holes up a little bit to make them fit. Definitely not cheap but the look nice.
There is a guy here in NJ that has 2 from his 1986 V that I can put you in touch with. They were in decent shape, the only parts missing were some flange areas that are covered by the trim anyway. Mine were the same faded yellow color as his which I painted and they look brand new. You can see a pic under the posting
http://www.wellcraftv20.com/communit...t=10985&page=2
Welcome to the site by the way!!