The fissure is still growing and is about 4 km now, and I’ve been following along the national radio RÚV[1]. There is a map of the fissure and the live stream angles there[2] (hopefully the link works). Based on it looks like the southern reach is near Gálgaklettar (N 63° 52' 4", W 22° 24' 20") and the northern reach is passed Stóra Skógarfell (N 63° 53' 53", W 22° 21' 39").

1: https://www.ruv.is/frettir/innlent/2023-12-18-gossprungan-or...

2: https://dankxip1iu8u9.cloudfront.net/eyJidWNrZXQiOiAicnV2LXB...