I took this rig for a 10 miles test drive at 55 mph. I went 5 miles then turned around and tested back the other way. I registered 23.6 mpg going south and 23.2 mpg coming back for a average of 23.4 mpg. Not bad as I can get 26.4 mph easily with the lid closed. This means that having the lift gate open like this is a 3 mpg penalty at least maybe a little more.
When I enclose the sides it will be a whole different creature. No point in this rigging if I am not towing a trailer. The question is will it improve trailer towing mpg over having the lift gate closed in its normal position? The sides ought to offer more support if I can get a good fit after hardening the foam board and U Bolting them to the the struts.