When I saw that done years back by contractors making up prototype SMD boards by hand, they used sticks of a wax/glue stuff and melted it with a soldering iron. Hot snot glue wasn't a common consumer thing then.
Why not try hot snot glue sticks and melting a bit on with a soldering iron? It seems better than the glue guns, which dribble all over the place. It's probably better if you have an old soldering iron or soldering gun you just use for non-soldering purposes.
Beeswax melts at 62-63C, so that's a possibility, if it's easy to get.
Silicone sealant is used a lot in electronics, but I recall it's an acid-free grade.
There may well be a product specially for the job, but you'd have to order it, and you don't need much.
I'd try hot snot and a soldering iron.