I also just recently switched my bp into a tub, and I had problems with both the cool side temp being too low and the humidity being way too high. I ended up having to add a bunch of holes and switch to aspen instead of coco fiber, and now the humidity is perfect but the cool temp is still a little low. Right now I'm using my old infrared heat bulb w/ clamp lamp to keep it several inches above the plastic lid, and it's keeping it around 80, so I would suggest trying a heat lamp or a CHE (just make sure it doesn't actually touch the plastic obviously).
I also saw a great idea once for a cheap radiant heat panel. I tried searching for the thread and I couldn't find it. From what I remember, you just take another UTH (controlled by a thermostat) and put it on top of the plastic lid instead, then cover it with a large piece of corkboard or some other kind of thin wood, and tape it down. It should be insulated and help raise your ambient temps. I'm thinking of trying that at some point, probably when winter rolls around again.