Bought a 21 1/2qt canning pot with rack from Walmart the other day. I found that there were not enough wires across the bottom of the rack to support 1/2 pint jars. They would keep falling over. Not good. Went back to Walmart, bought a cheap 12" by 18" white plastic(polypropylene) cutting board. Cut out a 10 3/4" diameter circle, using my bandsaw. A jigsaw or thin handsaw would also work. Drilled a bunch of 3/8" holes using a spade bit. Worked much better than a twist or bradpoint bit. Cleaned up the burrs left at the entry and exit points of the holes using a 3/4" countersink bit. I found that the plastic floats, so I drilled four smaller holes around the edges, and used some small 18ga aluminum wire and secured it to the outermost wire of the rack. You can buy a spade bit and the wire at any Ace Hardware.

I made a batch of hot pepper jelly several days ago, and the rack worked great! I hope someone finds this post useful. That's what forums are for: sharing information.