It's definitely doable. There are plenty of tutorials out there on the internet. But like others have said, there's plenty that can go wrong if you don't get it right (a fire that can't be put out).
Personally I would consider building one myself, but there's no way I'd keep it in my house (it would be going in a fireproof metal shed at the bottom of the garden or similar). And tbh, that's probably more trouble (and expense) than just buying a commercial one.
I mean, ultimately you don't. And there's always going to be some risk (ideally I'd definitely put any home battery in a fireproof location). But a commercial one is at least likely to have high quality welds, be wired properly, and have some kind of QA procedure overseen by someone who knows what they're doing). If there's a systematic problem with the design you are also likely to get notified via product recall, etc.
Personally I would consider building one myself, but there's no way I'd keep it in my house (it would be going in a fireproof metal shed at the bottom of the garden or similar). And tbh, that's probably more trouble (and expense) than just buying a commercial one.