>>2938780
If you want, you can add a small plastic container for the "inner part" of the nest, as filler, so that you save on grout.
Given the small amounts we're talking here (probably like 500g total) and the relatively cheap price of grout, I'd say don't bother.

One important point: You want the sand to model the channels to be wet, not soaked. I didn't dry mine enough after the cooking and it results in water pooling in the process, which I have to remove with a pipette periodically. Letting it dry and then slightly wetting it until you can form it is almost certainly smarter.

I've said I don't have anything better/else to do today, but I've gotten a call and an invite to do some stuff, so I will be going out to drink and eat with some friends. Will keep going once I'm back, worst case if it's too late tomorrow. The lower part of the grout can just start drying in that time, that's perfectly fine.