How do I fill this gap?

[Diydolly36]

Without using expanding foam. It's where the skirting board meets the floor. I'm going to caulk the small scruffy gaps but this floorboard has been cut short & I don't want to lift it now I've started painting the floor. I was thinking a gluey pva'd twist of newspaper tied to a piece of string could be poked through & pulled taut until it dried & then the gap caulked, but has anyone got a better idea?

I second caulk - don't get too technical with it, just squeeze it in and then smoothe it off.

To be clear, I have to poke something into the gap first as there's nothing behind just a void to the sub-floor. Caulk is fine for the front after I've managed to plug the behind, otherwise it'll just fall through. Any ideas?

Just plug the gap with scrunched up newspaper then use caulk over the top.

Piece of cardboard? I'd be tempted to cut the nozzle of the caulk into a large hole and see if I could just fill it with a thick squirt - it's so sticky I doubt it would just fall down. The gap doesn't seem huge.