Possibly another idea is to let you do either 2wd or 4wd not both, i'm booked in to do both but would be willing to give one up to let someone else race, did do last year at southport, left the 4wd at home but then there was enough space for me to run both as people are in a frenzy to book in then don't even turn up
to late by then though.
so maybe a pre pay system is a better way of making it more likely that people will turn up after they have booked in.