This hatch could maybe used on the roof of a carat, the smallest one they make is 10 x 10 inches or 250 mm x 250 mm... its only about one inch above the deck or roofline in this case, and is glass not acrylic which can be either tinted or clear. The frame seems pretty robust. Only thing is, don't know how to get this in the US.

http://www.marinemegastore.com/product.asp?pf_id=MER_35050&jump=0&cat_id=DECHAT001&cid=IUR9N111DHLMRMXXKXQ88M3C1P62TD51

My idea is to have these under the solar panel and / or cargo, which will both rest on those newer Thule aluminum racks. This way I can leave it parked to ventilate and if it rains the vents are covered. The vents will just be opened a crack but thats better than suffocating heat - just opening up the sliding door a crack really make a difference, but those side sliding windows are really worthless. Intend to have the front heater fan blow in reverse when parked...