The other approach is to use a second set of standard size fenders to widen the original set. Determine how much wider you need to be, then slice both fenders length wise, making sure to leave the additional amount on one of the fenders, then weld the two together.
Probably a lot more work to do than to describe...
I'll confess I've seriously considered this approach more than once and even have a few extra fenders just because I expect some day to actually do it.
