For my guestbook, I put Disqus on just one page and let that do the work. However, I'm not sure if that works on neocities hosting, and you have to specifically opt out of it collecting some "non-identifiable" info for targeted ads (grimace). Valid to have 18+ art tho, and it's great that neocities can be place that you can safely share it! ^^
note to anyone checking my page: i seem to be facing an issue where no posts can be made to my profile by myself.