Weld in is probably best but for a quick fix, chip off the rust, treat the rusted areas with rust converter, pop rivit in some light weight metal. clean patch area free of dirt, oil and loose stuff without using water and then cover the patch area (both sides) with Bondo fortified with fiberglass strands(aka kITTYHAIR). Then paint to cover well beyond the patch. This will get you down the road and will last a few years if done properly. The purists will complain about structural integrity and they are right but that area isn't the number one most important spot--unless you get hit there
My two cents worth.