Yet another base location post

I know there have been about a million of these posts before, but I guess mine is slightly different. So, what's one more?

I've enjoyed this game on and off over the years, but only just now got around to doing a serious playthrough. I have 7 decent fighters and all the research done. I may recruit a few more later on.

What I want to do is build a base that is completely self-sufficient. That is, everything can be produced inside without venturing out: food, medkits, weapons, every kind of armor and ammo, etc. Also, I want it to be in a location that is as challenging as possible in terms of raids and enemies, preferably with multiple types of enemies trying to assault my outpost. Basically, a real "fortress under siege" scenario. What would be the best place to do this?