Other suggestions:
Best one by a long shot is altering slightly the the BMG “freezer”. Tu B’shvat boys can only date girls 21+. To date younger they must wait till Pesach. (perhaps out of town 20 year old girls should also be allowed) It’s the boys choice – wait or date close in age.
Close to 400 boys with a personal incentive to date close in age. Now that’s a effective, implementable idea that doesn’t cost a penny.