If it were me I would go to harbor freight and just pick up one of their trailers for $140 bucks.
Here is mine which I use as both a trailer and/or a beach cart (kind of like the Cat trax carts they use on the Hobiecats)
I had it up to 60 mph pulling it with my wifes scooter. I can also pull it with her bike
It actually rolls thru the sand really well surprisingly, I just leave it on the shore just like the cat guys do with their cat trax wheels.
I made it in two separate pieces so it can either be used as a trailer or as a beach cart. Here it is hook up to the car.
I actually store my TI on the cart in the garage, it's actually pretty easy to pull the TI up onto the trailer (actually super easy), but when using it as a cart, if the cart had a handle up to the front it would be easier to load on the beach cart (I don't have a handle now but plan to add one). Currently when towing the TI with it as a cart I just clip the front of the bowsprit to the frame on the scooter (the scooter has a strong frame to support the basket in the back). If you made a handle to stick out past the front of the boat, that could double as the hitch ( I believe most atv's have a half in hole in the back for a pin hitch (like on garden tractors, well at least they used to I haven't looked at one in 20 yrs))
Here is a pic of the back half of the trailer (which doubles as a beach cart). The aluminum in the front is removed by pulling two clevis pins.
Here is a pic of the detachable front section, that would stay with the car (it breaks down and can be put in the back of the SUV)
Here is the whole trailer
I bought a utility trailer at Harbor Freight for $140 bucks (I think it was on sale at the time), the extra aluminum and hardware I think was around $150 bucks extra, and I built the whole works in 1 or 2 weekends with just a hack saw and powerdrill. I've been using a while now with no issues, and have quite a few highway miles on it now.
My plan is once the steel rectangular frame begins to rot (we only go in salt water) I will replace the steel with 2 inch aluminum U channel, the trailer currently weighs about 140 lbs, by replacing the steel with aluminum I think I'll take off nearly half the weight (the aluminum version will be much lighter construction.
The nice thing about buying an already licensed trailer (comes with a title and all the safety specs from Harbor freight), you don't have to have it inspected to get a trailer license (if you ever plan to register it and drive it on the roads).
Just some possible ideas for you