The conservative radio stations could promote a lottery contest. Trump believers would be encouraged to send in their name, address, a phone number and a dollar along with a guess as to the date when the first check will arrive from Mexico to pay for the wall.
The person coming closest to that date will receive all the money. If no check is received from Mexico in 2017, the money will be awarded to a deserving charity. The contest can then start new again in 2018, 2019, or as long as it takes. Charitable organizations could end up being flush with cash.