I don't think its a good idea to install windows other than the vertical position it was designed for. I suggest looking in the Velux catalogue. I suspect they offer an opening skywindow near the dimensions you speak of.

They are expensive but well designed and well worth the investment.

Kind of like the MB of skylights.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.