Funny you should mention that. Pool contractors only like to use round skimmer boxes, though there are square ones available and would be a lot easier for paver installers. I've never won this battle.
We use hand grinders and take about an hour on each to make them look really good. You can, and we have, rented a 12" core drill bit at a local rental store. They aren't cheap to rent, and even more expensive to buy, but it does give a real clean efficient look.
Bottom line is there is no silver bullet on this. It's patience, and hand grinders, cut off saws, and skill.
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