I've modified and lengthened my HF trailer since the picture was posted a few above. I actually bought a second kit (on sale + 20% off coupon) and added a 4'x4' section (it's now 12' long + tongue and much easier to back up) and used some of the leftover cross members to strengthen it a bit. Also had to weld the new section to the old trailer at the joints as it was a little weak with the extension. Constructed a modular second tier - 3 pieces - the top comes off by loosening 6 nuts (each side) on the u-bolts connecting it to the side rails and sliding it out, leaving the u-bolts in place and not MIS-placed. The side rails are connected to the frame with a turn buckle at each end. Loosen the turnbuckles and the side rails lift off with the chain and turnbuckles attached, again not to be misplaced. Road tested today with my buddy's PA-14 on top and it rode just fine. I have a couple little tweaks to do to tighten it up just a bit more, sould be an easy hardware replacement and relocation of the chain/turnbuckle points on the frame. Painted it with truck bed liner paint, second tier is mostly uni-strut (available at Home Depot). I did make one mistake, I cut the 2x4's about an inch too short and the seat on the lower kayak hit the PVC rails of the top tier. Not a big deal removing the seat when I have the second tier on, which will NOT be the majority of the time.
P.S. I removed the J-bar seen in the picture. Can't have them that high on the trailer so it's only going to be able to carry two at the moment, still thinking about a way to get the J-bars on to carry a couple more on their sides, if need be.
The J-bar has been deleted, I had it on to check fit with another PA-14 on top. But you can see the uni-strut used to form the top tier. I can take it apart myself in about 10 minutes, breaking it down into the 3 pieces. If I have help I can take it off in one piece in less than 5 minutes.