It's not a major modification, but I'm adding Box 23 to my copy of UK Deal or No Deal. The game came with 24 boxes, so designating one of the two extras as Box 23 isn't hard. Ultimately, I'd like to have a normal box 23 and the modified one used during Deal's Summer Party week a few weeks back (Double, +£20,000, Half, Quarter, Nothing).
I used three d20s for Bullseye, with each of two d20s representing the upper windows (1-5 would be increasing amounts for category 1, 6-10 would be the amounts for category 2, and so on). The remaining d20 would be the contract window--1-3 would be 1 question, 4-7 is 2 questions, 8-11 3 questions, 12-15 4 questions, 16-18 5 questions, 19 and 20 are bullseyes.
Points to Jake for mentioning Bezzerwizzer.