Hey Linda, I’ve actually been having the same problem with humidity that you are.
My cham’s humidity requirements are the same as Eustis’s. I’ve been keeping my basking bulb on all day, with the exception of it going off for a half hour for a mid-day misting. This is what has been causing levels at the very top of the cage to plummet to 33-30% (sometimes even 29%

). I’ve been keeping it on so that he has ample opportunity to get warmed up, should he feel like it. When the basking bulb is off, I have 0 issues getting RH to 50%. The problem with this is that ambient temps at the top get down to 67-66 F, which my cham seems cold at (he’ll flash darker colors and try basking under the plant light for a while instead

). Since the weather has been cold, I’ve dealt with low day time humidity. I‘ve been concerned that maybe my cham can’t properly shed because of it, and that may be the case (guess I won’t know until I change up the basking light schedule and let him sit in that 50% for a while). His colors have been dull and muted, but his urates look normal and RH levels are always pretty high at night (85% or over). I showed recent pics to the breeder I got him from, and they seemed to feel pretty confident that what he is experiencing is winter funk.
I use window shrink film on all sides of my Reptibreeze but the front and service doors (I had actually bought sheets of coroplast to use instead, but I still haven’t gotten around to putting them up as the shrink film is actually pretty effective). I think you also going this route should work pretty well for you. You can try covering the left, right and back sides with shrink film like
@Beman suggested, leaving it off the front door. Try that first and see where that gets you as far as humidity levels go. I’ll also second
@dinomom ’s suggestion to leave the basking bulb off for the rest of the day (after Eustis has warmed up). Being a montane species, I think he can definitely handle cooler ambient temps.
Just going off of what I have experienced so far - it would seem as though hydration during drier months shouldn’t be too much of an issue so long as you are able to consistently get humidity up pretty high at night. You said your cage has been sitting at about 70% while Eustis sleeps, which is decent. Fogging should also help,
especially after you get that shrink film on. I’m sure you’re already doing that when temps allow, though.
@dinomom already mentioned a room humidifier (cool mist), which you might consider getting as well

I definitely intend to get one! 30% humidity isn’t that great for us, either. Anything lower than that and you’ll start having to deal with things like nosebleeds.. These dry winters are rough lol