DO NOT use pressure treated material for this.
Resins DO NOT adhere to PT material, and will release shortly after installation!
Also, you should not be cutting into any fiberglass material
other than the laminating layers and final top layer.
The Original Hull material should remain un-disturbed.
A router w/ a straight flute bit, set to the correct depth, will allow you to section the original "Core Material" into smaller sections that will make removal much easier.
A pneumatic chipping tool works great for this.
Cover yourself with Baby Powder to help keep the glass from entering your skin.
No need to use layers of 3/8" material, IMO.
I'd use two (2) layers of
Fir plywood that will make up the thickness required (maybe one layer of 3/4"... another layer of 1", for example).
You will be creating a
"Box Beam" effect when all pieces are completed.
(I.E., original hull/matting/plywood layer/matting/plywood layer/matting/ finished with Roving.)
This plywood need not be Marine Grade. Many factory installations did not use Marine Grade..... they opted for construction grade CDX. It will work just fine!
Make sure that the core is also Fir!
(Fir is one of the best that offers both strength, and resistance to moisture deterioration and lends itself to resins very well)
Oak, Maple, Birch, Oily woods, are all inferior to Fir, IMO.
Layout, more layout and layout.... accompanied with many dry fits!
It may take you as many as ten/twelve dry fits to get all pieces to fit well.
This is KEY to having a good job!
Use every available hole in the transom to pull the resin wet first layer into position. You may even need to make a few new holes, as hard as that is to bring yourself to do!
A straight edge, or "stiffener" can be used on the exterior to prevent the original hull material from distorting while the original core material is absent!
A hull/transom without the core material may flex.... the new core will stiffen the Un-Straight transom causing issues when the Volvo transom shield is installed later!
Second layer can be screwed to the first layer from inside!
Cut your transom unit opening last!