I have been incubating multiple clutches of carpet eggs for years with the over water system, no perlite or water crystals, the water is underneath a plastic mesh and you just have to be careful if you move the boxes. You can adjust the humidity by air vents so maintaining 70% should not be an issue. I sterilise everything (except the eggs) and use boiled water, never had any mould. Never had any problems apart from a failed aircon last year let it get a bit hot for a short time and some eggs hatched early but the hatchies were all good. I have used various egg containers including food steamers but now mostly converted baby bottle sterilisers, I always leave the clutch intact and find they all hatch together within a short period just as they do with maternal incubation.
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