Yikes, that is a lot of holes. I would go with newspaper but if you don't want to do that then try aspen bedding and make sure the water dishes can not spill. I would grab an extra set of bins for the winter before the lot changes. BTW, those humidity gauges are not that accurate. I would say if there is not condensation on the bins that the snakes are okay regardless of what the gauge says. Maybe try a different gauge?