If you're welding in a flush patch(no overlapping joints at the edges), I've heard you could tack weld a couple of flat washers to your patch, then lay it in the hole, then tack your patch to the body and grind the tacks holding the washer onto the patch to remove it. Then you can weld the patch the rest of the way in. Give it a try. Might work O.K. for your situation.
