one of Dad's early rural mail vehicles was a 63 Chevy wagon with AC installed after purchase by the dealer..provided marginal cooling with the large area, so he closed off the rear of the wagon with clear plastic... used a couple of yard sticks to make an arched support with the roof... ends fitted against the trim panels and held plastic firmly against the roof... used "chicago screws" machine post screws to hold the two pieces of wood with the right amount of tension... might work for you to partition off the rear of your vehicles with mylar