Welcome. It's fun to see people sign up for interesting reasons!
I would build your new trailer as close as possible to this, as low as possible and using as thin of materials as possible while still being structural. Even if you only streamline the rear end, it'll have much better pressure recovery (and MPG) than that topper.
Aerodynamic Streamlining Template
Don't know how far you are from the water, but have you considered towing it by bicycle and just having a storage shed at your house? Five miles or so really is not bad, even carrying a load.
Your trailer could be much lighter, too!
(I used to bike-tow my kayak to the local lake with a custom trailer made of copper conduit and harbor freight wheels... whatever works, eh?)