Don't know of any trick to make the towel work better. It's just the nature of using a towel - works but not ideal in any way and is just considered a "make-do" option. Rack is the ideal.

Dave

I use a perforated flat canning rack that covers almost the whole bottom of my large BWB pot. It took me a long time to find one (at a garage sale - an antique canning rack I guess), and a modern circular wire cake cooling rack for second tier processing. Those divided racks that can come with or can be bought separately for a pressure canner or BWB pot are made for 7 quarts and since most everything I make is in 4oz and 8oz jars, those simply wouldn't work; same for your 12oz jars. I find it does not matter a hoot if their sides touch in the BWB pot; I've never had a problem and I can thousands of jars a year.
You might want to measure the diameter of your pot and do a search on-line, cooking store, or on Ebay for round cake cooling racks. Stainless would be best; aluminum might be too light weight to support the jars if they have feet, but as long as the jars are not touching the bottom of the pot, that's the whole purpose of a rack. You do not need it to have "feet" and if the ones you choose do, then turn them upside down! I have two racks so I can stack a layer of 8oz or 4oz jars, and another rack on top of that for more 8 oz or 4oz jars for big batches.
Nancy

After doing a quick search found that several on-line restaurant supply companies have really inexpensive cooling racks - like less than half the price of places like Williams Sonoma and Amazon. Is there a restaurant supply store near you to save on shipping too?
Nancy

Whenever I try to use a towel in place of a rack, the towel bubbles up and my jars fall over.

Nancy,
I did not know that you could layer your BWB canner. I thought it was only done when pressure canning. That is really useful to know. Probably too late for my canning this summer but will try it next year.
Helen

I was able to buy a flat stainless steel canning rack from Amazon.

I spent years canning with a soup pot and a towel. A rack is a lot easier, but the pot/towel worked when I could afford nothing else.

Like Nancy, I've found that it doesn't matter if the jars touch the side of the pot. Someone here gave me the idea to wire old canning rings together for a makeshift rack, it actually works pretty well, especially if you put one of those cake racks on top of it so your jars don't tip.

So, in essence, I don't think there's any way to keep the towel from bubbling up, but it does work in a pinch.

Annie

I actually went to a local restaurant supply store and found these pizza screens that look like they will work nicely. Very low profile. Not sure if they'll rust, but for $4, I'll give it a whirl and see what happens sometime.

I also have some pizza screens that DH brought home from a job (they were getting new ones). Mine are a little big for my smaller stock pots, and they only work with my full sized BWB canner, which has a rack. That said, I do keep one for that rack, as it provides a lot more stability for the half pint and 4oz jars that I do lots of, compared to the wire rungs of the handle rack. I just sit it inside the handle rack.