Go to my website, 3rd page, bottom, to see how Jeanne and I do it. This has worked fine sailing all over Mission Bay.
http://home.earthlink.net/~leinfam/
The key is using the tail of the tiedown straps to tension the tube directly above the middle of the boat (between the two drain holes). The extra lines I ran (x-pattern) turned out to be unneccesary.
Note the strut between the masts. It is just notched on the ends, holes drilled in the ABS, and bungee cords wrapped to hold it in place. You could use 1" PVC pipe instead of masts, to secure the front strut. The strut is only there to keep the bows in line...they are free to swing up and down with wave action.
Regards, JimL