Wow, that is
heinous!
Maybe blunting that flat front, egg shaped, and smooth up the sides. I think you could do well with the existing trailer actually. Find some new axle bearings, take them apart and degrease them, put DFL and moly/graphite on all the surfaces and then maybe Slick 50 grease. Increase air pressure in the tires to max pressure or even 25% over (many ecomodders go 55 psi on a 44 psi tire)
A boat tail rear may not be practical, but vortex generators can reduce the drag behind a square backed trailer substantially, OTR tractor trailers use them these days
Good Luck, and why not try and increase the non-towing MPG at the same time? It will help when towing as well, if it makes your ZX2 more efficient.