I don't like the idea of adding the cornstarch and sugar to the boiling water, even if it rests.
The water will still be very hot, so what would prevent the cornstarch from clumping up? I would mix in some cold water with the cornstarch first, then boil the remaining water, etc.
"Bring the water to a boil in a large, heavy saucepan. Remove from the heat and let rest 5 minutes. Whisk the sugar and cornstarch together. Add the mixture gradually to the hot water, whisking until completely incorporated."