Discount Ramps also has a ramp length calculator.
Big Tex trailers makes a 14 ft dual axle car hauler that retails for as low as about $1800. It has a flat bed as opposed to a turned-down rear, so it needs longer ramps. According to the Discount ramps calculator, it needs a ramp at least 8 ft 3-3/4" long. Discount ramps currently sells their BW-10840-HD 9 ft ramp for $350 shipped and two would be needed for the Slingshot's width. I have been thinking about this approach, and carrying the folded ramps mounted vertically at the front of the trailer to act as a stone guard when towing. Over on the Slingshotforums, at least one member has had an aluminum trailer custom-built, but I don't remember what the cost was.