All good points Dave
I think I should have used the word "Contain" instead of "find" in the question because one really needs to know the payoff to 'Find' any overlay.
Also, some Harness tracks have very low handle pools which makes for wild swings in payouts.
I went with Trifectas because of their enormous take-outs which is a shark repellent. Sharks do make mistakes and overbet some horses but the overbet money gets spread over all other entries payoffs so any overlays are too small to bother with.
Trifecta also gets ignored by the Robots which need probable pays to be effective.
I may be the only Trifecta vote.