You’re the only person getting lower than 80 F temps. I recall @zendog placing something over the cable which cooled his a bit, but you should be very happy with 70-84 it is my opinion.
One guy I know with a callus bench he built long ago that is all plumbed and has antifreeze running thru it and heated with a small water heater said to me, ‘that’s too hot…’ meaning my temps should be more around 72 I believe he said. I said, ‘well, it’s working great up there at or above 80.’
You don’t want to have the plant material touching the cable and I think that’s another reason @ zendog utilzed that metal conduit or whatever it may have been over his cable.
Honestly, I thought 72 F was too low. I just remember vividly this other guy commenting how much lower and in the 70’s he ran his hot pipe callus.
Btw, he’s grafting easy-easy stuff with the exception of Beech. Lots of Japanese maples & Ginkgoes which unite with simplicity. And he doesn’t know any other grafting technique than Veneer. So he’s grafting a difficult Genus with a less than ideal technique. Same with Hamamelis which I hear are not so easy. So he messes with witch-hazel, too.
Also, btw, since I know you and know you’re going to graft a ton of persimmons would be likely, you don’t need to be scientific past what the original design of this pipe does. You’ll get near 100% grafting persimmons. They’re so-so simple.