Is it a mouse hole or a rat hole? It doesn’t really matter what the answer is, in all honesty; both rodents can fit through a very small hole, and can also chew on a small hole to make it a much larger one. Even if you thought that a rodent couldn’t fit through it, they will. That’s what makes these rodents so difficult to spot and then get rid of — they don’t need a wide gap to squeeze through.
If you can fit a pencil through a hole, you can fit a mouse through it. They don’t have fluid bodies, but you’d think they did by the way they move and contort their bodies to fit through the teeniest-tiniest of holes. Rats need a hole around the same size as a wedding ring. Mice need a hole that is considerably smaller than that.
To fill a mouse hole, you need a material that is going to be strong and hardy. You can’t really opt for something plastic or wood-based, because both of those materials are easily chewed through by a rodent. You can use sheet metal, however, and also steel wire wool. The latter can be shoved into a hole and then held in place using expanding foam sealant, cement, plaster, or even caulk. Those materials can be chewed through by rodents, still, but the wire wool/metal layer adds a reinforced barrier to hold for longer.
The best barrier for a rat or mouse is wire wool stuffed into the hole, cement to hold it all in place, sheet metal as a final barrier, and then more cement or plaster over that. Keep monitoring the area to make sure it holds, but you should find that the hole is fully sealed.