I came up with a way to make perma-filters using the UBlock Origin adblocker extension. Thought I'd share in case any other anons found it useful.
In the options of the extension, paste the following in the 'My Filters' section. Make new copies as needed and replace poster_id with the actual ID to filter. The second filter allows to filter posts that contain certain words.
8kun.top##.reply:has(.poster_id:has-text(poster_id))
8kun.top##.reply:has(.body-line:has-text(phrase))