Depending on the relative humidity in the room, and how much ventilation you have in the cage, even the sphagnum may not help that much. Try using a water bowl with a parge surface area over the heat (if you're using UTH/flexwatt) and/or reducing the escape routes for the humidity. I've kept ETBs in Neodesha 2' cubes before, and kept the humidity at 70-90% with newspaper substrate. I used a large cat litter tub for the water bowl (took up about 75% of the bottom of the cage) and one daily misting, and that was in dry Utah air. I know some breeders raise their ATB neonates in large jars with water in the bottom third of the jar and perches up top; no substrate.

-Evan