Yeah, do avoid Zip Ties.

I use LiFePO4 batteries inThe Honey Badger and what I find works is putting the batteries in a small LiPo bag, velcro from that to the chassis base plate and then having a removable metal strap over the top of it. I did previously also use Tupperware boxes fixed to the base plate to hold the battery + LiPo bag in place, padded out with shopping bags.

Battery specific rules are in the build rules so I'd advise looking at them but for LiFe batteries, even the fuse isn't a mandatory thing but you do really want one in as it prevents all the expensive electronics from potentially going pop.