Here are some pics of what I have:
This is the whole trailer.
Here is the removable back portion which can double as a beach cart. Two cleavis pins is all that holds the front half to the back half.
This is the front half of the trailer, which I would leave attached to the car (or leave it back at the campsite if pulling with the bike or motorscooter).
Here I am backed up to the water, I don't dunk the trailer in salt water, I just slide the boat off and on at the shoreline, it's actually very easy. It slides quite nicely on the PVC tubing mounted on 11 inch centers.
This shows the cleavis pin, and the extra brace I had to add (just in front of the cleavis pin) to re-enforce the end of the 1 inch sq aluminum tubing. That's my new wing jib sail all furled up on top of the boat.
Here is a view of the underside of the additional brace (1 1/2" aluminum angle aluminum)
Here is a closer view of the V holders that hold the bow, I lash the ropes over the bow to hold everything tight in place. The spray skirts are plain old patio screen material, it folds over all the sails and rigging, and keeps everything from blowing around on the highway. My jib and spinnaker sails remain attached to the boat all the time, along with all their control lines. I just lay them down for transport. The mainsail is slipped out, and laid over the center of the boat, and held down by the straps and spray curtains
The 1 inch sq aluminum is not super strong, so I slide 3/4 sq steel tubing inside the aluminum for extra strength. I then filled the ends with epoxy so water can't get in there, which also re-enforces the 3/8 dia cleavis pin hole quite a bit.
If you look at the pic of the underside, you will see 2 white rods sticking down thru the boat. These are PVC rods with aluminum slid up thru the middle (for added strength) that go thru the scupper holes to add additional safety just in case I have to stop fast (don't want the boat in my back seat), (probably overkill, since I also have a brace on the bow).
If I ever do it again I will use 1 1/2" PVC instead of 1/2" PVC like (Kayakman7 uses).
Once the steel frame starts to degrade (and it will quickly because of the salt water). I will replace it with 1 1/2 or 2 inch aluminum channel, which should bring the weight down from a little over 150 lbs to I'm hoping 75 to 100 lbs.
The receiver bar and hitch is what came with the trailer, I was careful to keep all the legal stickers in tact. I just extended the wiring harness and am using the light system that came with the trailer.
Of course it is all licensed and registered, no inspection or certification needed, because it's a commercial utility trailer.
I left all the original stuff in tact so I can actually use it as a utility trailer, just by moving a few bolts around, for hauling junk, or my wifes scooter.
I replaced nearly all the fasteners with stainless steel fasteners, I also clear coated pretty much everything. Another thing I did was filled over the top of the leaf spring with silicone roofing tar to keep it from rattling. Basically stuck a baggie in there and filled it with the silicone, it actually rides nice and quiet.
Any place the boat touches I covered with that insulation foam tubing they use around water pipes (Chakika's ROID foam), then covered with electrical tape wound around (like 10 speed bike handlebars), seems to work quite well. The 3M electrical tape holds up quite well to the sun and weather (you have to use 'only' the black tape because the black color tape uses carbon black to make the color, and carbon black blocks 100% of all UV).
Those angled braces in the front are just 1/4 x 1 inch aluminum bent then covered with that same pipe insulation, then wrapped in electrical tape. Holds the boat bow very tight.
The total cost was around $250-$300 bucks, and a weekend of labor to build, only tools used was a hack saw and cordless drill, got all the materials at Lowes.
The only issue I have is my roadster only had 3-4 inches ground clearance, and I bottom out at the boat launch if I'm not careful.
Hope this gives you some ideas.