For BFL13 et al... FLA vs AGM vs Li vs heated Li

I have read all the posts in this thread with great interest. Up until the last few posts I was going to ask if anyone has built there own 12 v 100 amp hr LiFePO4 battery. I wasn’t even aware that it was possible until watching a YouTube a few days ago. Apparently it is possible to assemble a lithium battery pack at different levels of complexity, from spot welding the individual cells together to buying already assembled cell packs.
It would seem to me that assembling already assembled cell packs adding a bms soldering the connections placing it all into a container would not be all that difficult, saving hundreds of dollars. Has anyone done this??