An update:

I finally finished the trailer - I bought a 4' x 6' utility Snowbear trailer and put 39" high plywood walls around it to box it in. We cover the top with a tarp. I took it fully loaded to the cottage for our holidays. The car drove like it wasn't even there. I still used the overdrive and did not find it shifting in and out between 3rd and 4th. My mileage was only slightly worse than usual, nothing major.

Cheers,

Gregg