For overwintering my Saturniid ova, I'm using a 3-inch tall wire stand fitted with a small cup made of aluminum foil perforated with pin holes (to encourage air circulation). I put the stand in a small (4-inch tall) thick-glass jar with a screw-on lid equipped with a rubber seal, for an air-tight fit. I store the jar in the fridge.
I'm using the cup and stand to keep the ova high and dry and chilled, and away from the bottom of the jar and any (mould-inducing) condensation that might form there. To guard against desiccation and to introduce enough humidity to stop the ova from drying out, without creating obvious condensation, I include one quarter of one drop of water.
After cutting a 1-inch square of paper towel into 4 smaller squares and reassembling the whole (so that the cut pieces are touching), I add a drop of water to the middle (so that the drop divides evenly among the pieces). One of the pieces (with its one quarter of one drop of water) goes into the jar before the lid goes on. I check the jar and its humidity periodically.
The (metal) wire and perforated foil should maximally conduct the cold and promote chilling, and at the same time be resistant to growing mould.
I'm including a photo of the stand.
