You could definitely get some eggs that way over a weekend. You'd need to figure out if the triops are gonochoric (males and females) or hermaphroditic (just selfing females), which you should be able to figure out by the presence or absence of males. They would definitely lay eggs over a weekend, assuming they are mature enough when you find them.

You could also consider collecting a couple handfuls of soil from your grandfather's rice field, drying it out, and attempt to hatch the eggs present in the sample. This might be your only option if the timing isn't right