if your willing to "cheat" a little and use a third party tool you could put your worlds seed into the map viewer at
https://valheim-map.world/
and see where the closest boss spawn point is.

If your not comfortable doing that then there really isn't anything you can do except keep looking. The stone towers, villages, "ston henge" and any other structures in the Plains have a chance of having the rune stone that showes you the nearest boss spawn. I had a similar problem on my first world, searched dozens of plains areas and didn't find the rune stone, then I stumbled on a small "stone henge" protected by two Furling Brutes and there it was. And the nearest boss spawn was on the other side of the frking map.

Good hunting and good luck o7