I was thinking about external power (not sure, just yet, whether I want it), and saw that the two links specified in 2017 were not longer available. Obviously, any Amazon link provided will (probably) eventually become obsolete.
Anyway, I found this contender for the battery pack: https://www.amazon.com/gp/product/B07TMYSSZP
But haven't yet been able to figure out the connection plug. I'm thinking about getting an accessory plug from the iFlyGPS store and just wiring one up myself, but if I didn't have to do that, I'd be happier. So, the question is: is the 740b power plug the same "3.5mm x 1.35 mm" you said was the plug for the 720? If so, I can poke around Amazon for a universal connector set that has that.
(Side note: since it's been a couple of years since you sold an external battery pack, you might want to think about removing from the iFlyGPS web site, the manual for it.)