These instructions will undoubtedly be baffling; perhaps making a drawing as you read them will help. Call me @ 307-733-4793 evenings and I'll attempt to answer individual questions and email you pictures of what you need clarified.
This is a verbal description of dimensions of the trailer Hobie sells for hauling 2 assembled Adventure Islands. People on this list have wanted to adapt trailers they already own to have this capability, and have asked details to keep their projects on the right track.
The Hobie trailer is well built and specially designed for it's job. It's made of galvanized steel, bolted together. It took my dealer a long time to figure things out; I'm glad I didn't have to assemble it. It has 2 pillars rising up between the kayaks, which form the foundation for mounting turn signals, license and optional storage box.
Buying 2 sets of cradles for the boats was expensive (I think about $300 apiece), but they're perfectly proportioned and cushioned. It would have been difficult to do this by myself. A friend bought a pair and easily mounted them to his existing trailer. The cradles actually "scrunch" the amas closer to the hull than they "want" to go. Making your own, you'd have to strap the amas close to the hull and then figure out the profile at the rear cradle. Also, the cradles are made of fiberglass, which gives a little to accommodate less than perfect boat placement. Good luck if you try this yourself. Note: The front cradle is "U" shaped to support only the hull. The back cradle is kinda "uUu" shaped to support the hull and amas.
The bottom front and rear cross braces (which the boat cradles are mounted on, are centered 5'6" apart. These cross braces are 6'7" long. The outside amas end up sticking out another 6" from the end of the front cross brace; this is a wider load than your vehicle.
Two pillars (3" by 2" stock) rise up 24" from the middle of the front and rear cross braces. They've had bases welded to their top and bottom with holes for a pair of mounting U bolts. Each pillar has a nicely padded "U" shaped cradle 9" from the top, left and right, to hold masts, The masts are easily strapped to these cradles with a welded chain link providing a place to knot a permanently mounted strap. Note to do-it-yourselfers: It wouldn't be as elegant, but you could simply use wide NRS straps to cinch the masts together tightly up high and out of the way.
A 3" by 2" piece of metal stock runs parallel to the trailer between the pillar tops, extending 9" behind the rear pillar and 1" ahead of the front pillar (allowing space for Ubolts). The rear of this piece between the pillars has an upside down 28" piece of wide "L" stock steel. This is for mounting the turn signals and license plate. The storage box you may choose to add isn't bolted to this, but the rear of mine rests on it.
Upper crossbraces, whose only use is to mount the storage box, are 1" by 3" stock (the 1" height raises the box above Ubolts elsewhere). They're 20" long. The upper cross braces are situated 22" and 58" from the front of the front of the 3" by 2" stock running between the pillar tops. Note to do-it-yourselfers: Having the upper cross braces a little longer would have expanded my mounting choices, but you don't want these to extend past the bottom edges of the storage box.
Sterns of the AI's extend about 5' beyond the bottom rear support. That's why the the lights are up higher than usual turn signal trailer placement on the bed of a trailer; this prevents the lights being hidden by the boat's length.
The front of the ball hitch extends 7'1" from the center of the front cross brace. These boats are sooooo long you need an extra long trailer tongue to accommodate them. There is only 1.5' of extra space from the bow of the boats to the front of the ball hitch.
I have the largest available Yakima Skybox bolted to the top of the the upper cross braces with 4 1/4" stainless steel bolts, nuts, washers and lock washers. While 1/4" bolts seem lightweight, they match the holes provided by the manufacturer. The 20" length of the upper cross braces was less than what Yakima recommends, but worked out OK. The last thing you'd want would be Yakima crossbar pipes sticking out from the sides of the box; you'd poke your eye out in low light. This isn't the official way to mount the Yakima box, but was done with their knowledge and has served me well. I changed lock cores to match my other Yakima gear.
My Yakima Skybox has plenty of room to hold life preservers, pedal drives, a few tools, dagger boards, a roller picnic table and 2 large comfy nylon chairs. Never one to be burdened with too much money, I added two solar lights I've never needed to use: http://www.sollight.com/products/lidlight.cfm
* Turn signals and lights need to be up high above the boats, since the sterns extend 5' past the end of the trailer. Lights at the trailer bed level would be obscured.
* When dragging the AI's onto their cradles, you must severly point the bows out away from the trailer. This drags the inside ama against the rear pillar and forces the ama to "nest" tightly into its rear cradle. The hull then drops nicely onto the front cradle. Of course, the boat is then strapped to the trailer over the cradles.
* This setup is heavy enough to make a trailer jack almost mandatory.