Looks good.
With a flat transom and easy access like that, wood was the way to go. V20s don't have a flat transom.
A little confused about the "marine grade' comment. Looks like you have marine grade there.
Marine grade refers to Exterior(glue) AB grade kiln dried Douglas fir. Douglas fir is denser than pine, which is what most home construction plywood is made of. The AB means that one face is clear of knots or repairs and the remaining layers have all knots repaired. This means there are zero voids in the plywood. Also there are 7 layers vs 5.
Some people mistake the run of the mill green pressure treated plywood for "marine". It is typically BC or CD grade pine.
While you CAN get Marine grade pressure treated AB fir, it is used for bulkheading, docks, Deck under carpet, it would not be used when fiberglassing.