My suggestion for what it is worth, is to have about a three meter length of tube galvanized and then cut sixteen pieces at 45 degrees with the long length at 150mm. I suggest sixteen pieces so that you can brace all four corners on the two ends and the back panel, and the four corners of the horizontal frame that supports the tank. This would leave the front completely open and the frame braced in all directions. You can then grind away the galvanizing on the frame and the braces, local to where you weld these to your frame. Clean the frame after welding, and paint the ungalvanised areas with cold galvanized paint. It is an acceptable method of alteration in the construction industry, so it will work on your frame.