I have also run across this in my battery research and I might feel brave enough to try and build my own battery as I can skip the soldering minus the major wires to get power in and out of the battery.
I really like that you can find and replace individual cells as so often with larger batteries just one cell goes bad but the whole thing “breaks”. Here you could even be on the road and a few minutes with a volt meter you could find and then easily replace the one bad cell. The Lifepo4 batteries put out 3.2v each so 4 parallel packs of whatever size will put out ~12.8v. I just need to calculate the AH per X batteries and then look at battery box options once it was all shrink wrapped up. If you buy the cells in batches they get a lot cheaper and I have read a few people have had luck with $1-$2 per cell on ebay, though going through there has been iffy for me several times with poor knock offs etc.
Sorry for all the posts, I can’t figure out if/how I can edit my reply from earlier to just add more info.