I think it would be unnecessarily expensive and heavy (not to mention a lot of work) to build that all up with glass. A properly bonded dry core and a new top skin will give you back the stiffness you'll need. Plywood should be fine, just a bit heavier than original..new balsa or foam would work too..
If the budget allows, I think I'd buy a 4x8 sheet of 1/4 'teak and holly' plywood (around $250, IIRC) and cover the whole area with that (after rebuilding the liner), with cutout 'lift outs' in the areas of the original bilge access boards. A single sheet would probably do and if you're really careful with the saw you can probably cut the sections out and maintain the grain and continuity, with the kerf giving you the right clearance for easy bind-free liftout. Epoxy seal the underside, and glue the 'cutouts' to appropriate plywood substrate so everything stays level.
This also saves you the trouble and effort of getting a perfectly 'fair' surface on your rebuilt liner top skin..
For the liftouts you can simply drill a clean "finger" hole (use a forstner bit) or install lift rings. Should be obvious which of those options saves you a few bucks!
A nice coat of varnish (shame that Ultimate Sole is not longer available) and your sole will look better than new!