Hey Jason, I’d say you’re doing all of the right things. I’d suggest sitting everybody down for a meeting, have maybe 10 choices of ‘prizes’ worth about the same monetary amount that your willing to give up (probably equivalent of 8-16 hours pay) and have them all vote on what they want to win. You could even keep two or three of the top ‘prizes’ and rotate them. That will get them more involved and probably start a frenzy of people wanting to win. 

Then, of course you’ll want a sweet wooden plaque that you can have the winner’s name on a bronze tag or something so each month a new winner goes on the board. Every year start a new one. Put the winner for the year at the top around Christmas! 

Think of 10 things and have them choose. 

Good luck!