P-clamps, particularly steel ones, are pretty strong, especially in the size you will need. I don't know if low-riders will work but a taller rack using the hole in the fork crown (for a caliper brake or a fender) as an upper mounting point should be pretty strong.

I would not weld anything to that fork. The metal of the blades is thin and the weld heat will compromise any heat treatment given it by Trek and is likely to distort them too.