(1) Done over 50 yrs ago, by both Clark, and Walker Foam. (2) Discontinued immediately. A nightmare for shapers and thier tools. (3) Replaced by glue line (resin) stringers, colored paper stringers, and 1/16th inch wood stringers. All in attempts to make lighter and lighter boards. Don’t bother going down that road.
You can buy PVC sign blank material in 8’ and I think 12’. You can get a lot of stringers out do a 4x8’. I use Sanderly Ply 3 layer. Comes in 4x8 sheets up to 1/4" thick. Similar to Poplar Ply which will also work. For anything over 8’ I piece it together with a V joint somewhere in the front third. It is easily glued up If you glue the scarfed stringer to one side of the blank. Then wait until dry and glue the other side onto the stringered half. Glue lines are stronger than foam, but not as strong as wood. There are "Construction Grade foams that make good liteweight stringers, but can only be glassed with Epoxy.