My trailer is 6x10. Yesterday we tried cramming the Gemini in diagonally -- and it fit! We'll probably go with that for long trips.
For shorter trips I love your idea of putting a rack on the roof of the trailer. Do you know anything about what sort of companies install those things? The trailer is by Wells Cargo, and they said they don't do racks -- at least at the time I bought it. I wonder who I might call to put a rack on ...