How can I fix my muffler end cap?

I took a ride this morning, and after all that time I couldn't get it off, the end cap fell off on its own. I went back to get it, but right as I got about 100 feet away a street sweeper came and ran over it. Didn't suck it up but now it's all bent out of shape.

If I can get a piece of steel, drill out the right size hole and have it welded on, would that be a good solution? Would it have to be rounded out or would flat be just fine? Hopefully I can find a big washer that will work.

I've heard about the can thing, but I don't see how thin aluminum would stand up to the muffler heat. I also couldn't weld it on, and I kinda want to finalize the muffler. In other words, I want to end the worry of it or parts of it falling off when riding. I just want to freaking ride already!

I will probably just go with a piece of steel or a big washer. I just wanted to make sure there wasn't anything about the stock end cap that was important to engine performance.