|The ACKS Attack Throw Assistant|
Align the arrow over the zero with a character's modified attack throw. The bottom numbers show all the possible results of the d20. Look at the number above the rolled number to determine what armor class was hit.
In the one I made the numbers 1-20 only wrap about three fourths of the way around the cup on the bottom row, and I stopped at armor class 13 for the top row.
If your preferred game uses descending armor class (the THAC0 mechanic) the numbers on the top row (the AC cup) will go the other direction, and you will need a range of numbers that is appropriate (such as 9 descending to -9 or -10).
It would be trivial to modify for d20 or Pathfinder as well.