Hi Guy,
I have an LT-40 on floats. It is a great plane to put floats under. I did use the GP 40 size floats for the first few years but found that it was almost impossible to keep them from leaking and still stay light. My brother makes and sells foam core floats that I now use, light weight and they don't leak. See - www.geocities.com/planefunfloats
You can buy 1/8" x 1" aluminum bar stock at Home Depot. See attached pics for how my flying buddy made and attached his.