No need to weld a plate on to use to center the hole saw. Do like Cummins12V98 says. Clamps will work, but you can also add bolt holes to so it can be bolted down using the bolt holes for the existing Jack.
I did something similar when replacing both crummy uncovered gravity and city water fill connections on my trailer with ones that had lockable doors. I needed larger holes to install them. I cut a small square piece of half inch plywood to use as a template. I used a 4 inch hole saw to cut a hole in the template. I then mounted one of the hatches to the template using the screws needed to mount it to the side of the trailer. I then removed the hatch and screwed the template to the side of the trailer over the existing hole. It stabilized the hole saw while I cut the first hole through the side of the trailer. I then removed the template and lined it up over the next hole keeping it in line with the first hole, screwed it to the trailer and cut the second larger hole. Worked great.
Cummins12V98 wrote:
Drill a 2-1/4" hole in plywood or steel and clamp over existing hole centering the template over existing hole. Drill away, ez-p-ze.
