Your case is pretty sharp.
One thing I love about the uBitX and BitX is the creativity. You can find 500 IC7300 or KX3 and they will all pretty much look and operate the same. You can find 10 uBitx or BitX's and they will all be different. These little rigs have some character and some soul to them.
Yeh I use cheap Westinghouse brand AA Ni-Ma 2000 mAh batteries. I think I found a deal where I got 40 batteries for $20 back in 2015 and I have only 2 batteries go bad since. This is the battery holder I use
It has a plug on the short cord that fits perfectly in my radios and an on and off switch that I use then they are not in use. They were a few bucks on E-bay and I take a few with me when I go operate portable. A single pack is not going to last all day long and it is what it is so you have to have realistic expectations on them but they can do the job. I don't know how well it will work on my uBitX because I suspect that the Nextion is a bit power-hungry.
I have no experience with the 18650 cells but giving them a quick look shows a lot of options for them.
4SQRP # 1080