How best to fill void under spill-over spa spout

[wpbucher]

Hi,

Pool build underway. I have a fiberglass spillover spa on the edge of a steel-walled vinyl liner pool.

I noticed that the PB has not filled the void underneath the spa spillover "spout"?

This is the first time he has installed a spillover spa.

I can reach my hand underneath the spout and feel the smooth metal of the top of the pool wall and then rocks behind that.

What is the best way to fill that void?

Build up mortar sloped toward the pool?

Great Stuff foam and caulk?

Looking for the RIGHT way to do this.

Thanks TFP!!!