I know of
A few screws short of a hardware store.
A few cards short of a deck.
A few fries short of a Happy Meal.
A few peas short of a casserole.
A few keys short of a keyboard.
A few sandwiches short of a picnic.

so here is a new one
"a gate or two short of a full design"
whicht to conform would be
"a few gates short of a full design"

:)

Erik