How do we keep a porch with a fiberglass roof cool?
Replace the new roof. Clearly you chose wrong in a transparent untinted roof. It is possible that a stain/dye of some sort (applied to the underside of the roof) might be able to block some sunlight, but I expect that it will bleach out far too quickly from that sunlight.

If replacing the roof is not acceptable, then you might need to add vents to encourage more circulation.