Would this one work?

I recently installed a home refer too (bottom coils) and instead of sealing the vent I built a baffle that directs the hot air out the vent. I taped over the louvers in the rest of the vent to stop outside air from coming in. Have not decided what I'll do in the winter in WPB as most days are more like summer than winter.
OP, I would tape over the louvers in the vent for now and see how it goes assuming bottom coils. If on the back I'd leave it open like DrewE suggested.