Rudy wrote:
Thanks for all your responses, thank god for epoxy, since there won't
be any load on the clamps except compression I think I'll just glue
them in place.

thanks again, Rudy

I wouldn't use epoxy. I don't think it will hold up very well. If you
aren't confident about welding them, you could make some kind of bracket
which goes around the frame of the C-clamp and clamps it to whatever
you're making. Granted, it's more work, but it'll be much stronger. If
the clamps aren't under a high load you could consider drilling bolt
holes in the frame of the clamp, but this isn't such a nice solution.
What size are the clamps and what is the project?

Best wishes,

Chris