ewwww. really?
some of these ideas are just crude, and the stock kit tanks look like poo on an o.c.c. frame in my opinion.
get creative, make a new, better looking tank and come up with a hidden mount.
seriously...plumbers strapping?
brackets screwed to the top of the frame?
doing it this way would set the tank tunnel up higher than the top tube of the frame and the tank would be "floating" above the frame.
sorry, but there are other ways to do it that would look hundreds of times better than some of these ideas.
here's my o.c.c., Jesse James bicycle tank (that i made fully functional smoothed out) with a hidden mount that I made.