Encouraging parent volunteers to meet obligations

I volunteer with our local nordic club on the board. For our kids ski programs, it’s mandatory that parents help out with two sessions a season. These aren’t coaching positions, it’s help with setup, takedown, snacks etc. We have tried a using a financial penalty in the past (a $$ deposit made with registration that is refunded at the end of the season if they meet their obligation) but the refund process was manual and for 200+ kids, it ended up being an huge administrative task for our club manager to process all the refunds. We use Zone4 for registration and for those unfamiliar, it doesn’t not allow for refunds.

Has anyone found a solution to this problem that doesn’t involve taking a cash deposit?