So I don't know if anyone ever reads neocities comments but here's a tip that wasn't obvious to me: if you're editing the not-found page, make sure that the links in it are absolute links (with the entire address included). You can end up on the not-found page in different relative places (e.g. either https://your.site/bad-link or https://your.site/directory/bad-link) and it you need it to work in both cases.