Just found your cool thread today.
How about a rectangular frame from 2" hd pvc pipe long enough to fit you buckets into and approx 10" between the long pvc pipes then the buckets will stay put. You could strap them in with the cheap 3/4" rachet straps. Strap them on top of pipe frame for travel and then strap them in under the frame for water "surface" travel. You would not need all the trouble of adjustable brackets .
The rachet straps hold it all together and add a lot of strength. You could ad a 3rd pipe on top but would hinder the travel option of buckets on top. Fit the buckets tight together for added stiffness.
You would still acheive your "diy cheap and easy" theme. The buckets are a sound idea. I would have bought the grey colored buckets, unless they are too short.
Drew the sketch in a hurry, don`t think the third pipe is needed. If your brackets could be angled to the end`s of pipe frame or add bridge like frame that buckets could slide under for travel that your brackets need.
Could the buckets be epoxied together for more strength. maybe 2 section`s on each side to go under bracket bridge`s.
Just deleted my "Nose Cone" idea, not nescesary.
Stay with it, have fun.
PUMP YOUR BRAKES
HAVE TO LOVE WINDOWS & THE INTERNET
NEVER BEAT ON IT, GET A BIGGER HAMMER
"A MAN WHO SAY`S IT CANNOT BE DONE SHOULD NOT INTERRUPT A MAN DOING IT"