assembling diy hosts is pretty simple if you can solder tbh, there are plenty available and driver choice will dictate cell compatibility as much as host.
P60’s do tick all the boxes you mention, they are both easy to lego and bare pills are available and simple to assemble, my big problem with p60’s is their poor heat handling, not good for new hot rod emitters.
You might not like this suggestion but how about a p60 for educational purposes and a nice diy host for yourself, can I point you at this thread.
its a dammed good p60 sized host that does give you options depending on driver choice and builds a real nice light, I’ve been carrying it since I built it and will be changing a couple of bits this month, namely using a qlite driver from intloutdoor and a hi cri xm-l2 on copper, I would also suggest looking at the t10 host from intloutdoor but use a qlite and copper mounted xp-g2, I am told this builds a brilliant light and will be finding out soon.