I have a 12' portabote that I carry under my Cruiser. I'll attempt to post pics.
I used the carry rack from Portabote (the 2 metal braces intended to sling the bote on the side of the RV), and added a roller mid way between the racks (makes it way easier to load/unload). Hopefully you can see it in the pics.
I also built a box to carry the seats, transom, life jackets, etc, etc. I used a bed frame for the angle iron, sheet metal from air conditioner ducts, riveted on, with a lockable door on the side. Again, hopefully you can see from the pics, what I did.Iused longer pieces of the angle in 4 places, so it would bolt to the frame of the RV.
Since these pics, I also made a cover for the bote, from 1000 denier Cordura, to keep the road grime off. I'm trying to figure out something better for that, but it works, for now.
So far, I have several thousand miles, and several unload/load cycles, and all is good.
Let me know if I can give you any more info on my install.
This url SHOULD take you to the pics.