Plus when you are done pouring have a flat rock or brick to set down over the hole.
Yes, put a clear bowl over the hole with a rock on top and leave it there all night and tomorrow. Windex will kill them, WD40, ammonia, bleach or wasp spray with a straw on the can. I use a small amount of gas and a bowl. Maybe 4 or 6 ounces then cover and the fumes get them. Just don’t light a match after that, your ground can collapse like a sinkhole if the nest is big enough. If you’ve seen the big grey hornets nest up in trees, that’s exactly what the underground nest looks like if you dig it up.