[skyking]

There are a couple of ways to do what you want. You could form up a backing frame out of heavy wire, that follows the inside of your lip. Weld whatever you want to that wire to support your cross bar, attachments for the top of the spat, etc.
Then get some spring wire, and make spring pieces that arch over the tire and press this in place from the inside of the fender well.
Think of something like that, that gets pressed into place by a force. Remove the force and it comes right out. You can paint it up and then cushion it with rubber or something to keep from marring the paint or coating in there.