I'd use MDF rather than plywood. Because it's heavier and because it has a more uniform consistency.
Also, is it really fiberglass? Or is it something like Acousta-stuf
Loose fiberglass I think might not be such a good thing, but if it's somehow attached to the box so it doesn't touch the back of the speaker, that should be ok.
Do you have a link to the product from PSB?