Timbor is one used for insect repellant. Boralife sells pre-treated lumber and your local lumber yard can probably supply you with other brands. IIRC for fire

Timbor is one used for insect repellant. Boralife sells pre-treated lumber and your local lumber yard can probably supply you with other brands. IIRC for fire retardant a 7:3 mix of boric acid and borax dissolved in distilled water (at 80C to speed up dissolution) can be used. Borax reduces flame spread but can cause smoldering while boric acid suppresses smoldering. AFAIK any concentration high enough for flame suppression is plenty to deter insects and prevent fungi/rot. I can't find anyone selling this as a packaged solution with instructions so I'd suggest doing your own tests before trusting a random HN comment. This is all from memory so don't take my word for it.