This is a very easy variant and really adds a layer of plotting and sleuthing than you would ever imagine.
Play the game as normal except for the following.
For each player in the game minus 1 you add a reward pool of real world money.
10 cents (10 pence etc or I play with £1 each being from UK and suits my usual group).
When shuffling agenda cards follow the rules for stacking agendas but place 1 fewer Loyal cards in.
This will guarantee a Terrorist in your game and keep the Political Opportunist relatively high to mix their actions with.
If Loyal/Opportunist win, they split the pool equally.
If terrorist wins, they keep the pool all to themselves at the risk of being cut out of reward completely.
I find this really engages people to pay even tighter attention and when it comes to end game scoring / accusations, add in this other change...
Any incorrect accusations, will give your prize pool to the incorrect player you accused.
If however you guess the terrorist correctly, despite Loyal / Political Opportunist winning, only those who found the Terrorist split the pool.
Where the amount is not easily split, the player with highest VP gets the extra change.
If VP is still tied, shuffle their case lead cards and draw a winner to take the extra.
This isn't gambling cash, you the host put the money up, it doesn't have to be a lot, but the prospect of real world cash mixed with a great fun game really pulls people focus especially new gamers I've found.
In terms of how this money fits into the game story.
Consider it a pay bonus for the Loyal and a procurement of funds for dithering your terror!