Any parellogram would work since given four digit number n1n2n3n4, it is divisible by 11 iff n1+n3=n2+n4, and each ni is linear combination of the coords of keypads xi, yi, and thus (n1+n3)/2 = (n2+n4)/2
1000a + 100b + 10c + d = [a + b + c + d] + [999a + 99b + 9c]
= [a + b + c + d] - 2a - 2c (mod 11)
= (b + d) - (a + c) (mod 11)