I have Bill's 12x20" propane finisher:

http://wfmasonwelding.com/finishersbottlers.html

and finally got around to trying something that I thought others might want to try to avoid having to worry about flame hotspots, or non-uniformity in the heating when packing glass.

I had some full-size shallow stainless steel hotel steam pans from my days of boiling over an outdoor arch - these can be had for about $10. It just so happens that you can put the hotel pan on top of the steel frame and then set the canner on top and it all lines up pretty nicely! The steam pan does not sit down inside the frame, so the distance from the burner is unchanged. This just raises the finisher tank higher by the depth of the steam pan. This did a nice job of uniformly heating the finisher tank up even with relatively low flame.

The only drawback I saw was that you lose the ability to use the very nice tip-up feature. And while it may not look or sound stable, it was rock solid with 6 gallons of syrup in it.

I would think that Bill could pretty easily modify the frame to recess the hotel steam pan flush with the frame top so that the finishing tank could still be held by the frame corners and tip up, but this is well beyond my skill!