I just cut a piece of no-see-um netting to roughly the size and shape of the mosquito netting, and sewed it right onto the mosquito netting around the edges of the netting.
Unless the tent/canvas is new, and you've just had a little mishap. Or you're trying to fix it on the cheap. I'd replace the old canvas completely. Sooner or later, you'll just end up having a patchwork canvas, and a nice bus deserves better when the money is there.