A windshield and an aquarium are different things.
I actually have experience with making polycarbonate aquariums. We have looked at making Minibows out of polycarbonate in the past. It never comes out as clear as crystal styrene and other than R&D samples we tested, it's never been used in production. The tanks look terrible. And those are small desktop tanks. I couldn't imagine how bad it would look on a thicker, big tank.
A frame will only reduce the bowing at the top. But the center of the panel will still move out quite a bit from the water pressure. A typical glass aquarium will bow out in the middle about half the thickness to the full thickness of the front panel. The glass doesn't stretch or get longer, it pulls it in from the sides. Silicone allows a flexible seam which allows for this. For acrylic and polycarbonate which are glued with a inflexible seam, the bowing of the panel actually causes the material to stretch. If the bow is more than the material can stretch the seam will pop or the panel will crack. Which is why you need to use a thicker panel to reduce the amount of bow and keep the sides from popping.
Believe me, If polycarbonate was a lower cost alternative to making acrylic (or glass) aquariums. We would have done it by now.
