I never have a problem holding humidity with my tanks with screen tops. Maybe my house is naturally humid. But I just mist the tanks and put a towl over the screen covering most of it but leaving a little opening for fresh air to flow through and all my snakes shed perfectly every time. And I'm using aspen for bedding. But a good misting with a towel over top always works for me but if that doesn't work for you you could dampen the towel with some warm/hot water not too hot and put it over the tank as well to get it really humid.
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