If anyone did want to make the end cap domed, just get yourself a small piece of sheetmetal and some kind of sturdy bag you can fill with sand. I used an old welding glove (the actual real-deal item is called a shotbag) Get yourself a mallet with a rounded end, hold the piece of sheetmetal on the bag/glove and give it a few thumps. You'll obviously see some dents appearing.
The sandbag idea is to try and keep the dent you're making fairly uniform, as the sand won't displace very much as you hit it.
Once you've dished the sheetmetal a bit, move onto a nice smooth bit of concrete and again using a mallet with a rounded end start working the dent into a smoother more uniform shape. When you've got it concave enough hold the muffler tubing over it, scribe the circle onto the sheetmetal dome and snip away. Then all you've got to do is drill a hole in the middle and weld a flange onto the back of it, so it'll slip inside the muffler, or whatever method you have in mind of putting the whole thing together.
