I have to repair a hole (~5" in diameter, irregularly shaped) in a slab of bluestone, but I have to do the repair from below and am looking for advice on how to get it done. It doesn't need to be pretty - it's not a visible location - so I'm looking for any solution that will allow me to fill that area with any kind of ... well, any material that will seal up solidly.

Any thoughts on the best way to tackle this?

Many thanks,
jtf