Here is an affordable way to cut out the top from a industry standard 1/2 barrel (15.5 gallon keg). I used a 4″ inch cutting wheel (rated for stainless steel) mounted inside of a 4″ grinder. Later once hole cut out, I switch to a grinding wheel (not cutting wheel) to clean up the edges, link below to the youtube video.
You want to employ the safety shield that is installed in the grinder tool, don’t remove it.
Glide it along-side the inside rim of the keg (watch video), no template is needed. First go around to create a mark line in the metal, if you make a mistake you can correct it, go around and do that first, don’t cut all the way in. Once you are satisfied, then you can use the mark to easily glide along the metal and finish cutting it.
Youtube link to video is below: