You should add something to the top that is less breathable than the towels. Plexi, glass, foil wrapped cardboard, etc. That would help keep heat and humidity in. Leave a small area for air but I would close off most of it.
I use a mixture of cypress, repti bark, and eco earth. Cypress is good but Eco is great at holding humidity. The only thing I don't like is it gets kinda dusty once dry. I found using it mixed with the other two it takes away the dustiness of it but still gives you the humidity bonus. I use this in my GTP cage and it looks and works great, especially in a naturalistic tank like you and I both seem to like.
Another thing you may want to check out is the ReptiFogger. I used one with my BRB when he was in a similar style cage as yours. I paired it with a HygroTherm and the two worked very well creating the humidity I needed. It also looked really cool when the fog would coming rolling into the cage. If you go this route a tip is a regular 2 liter bottle will fit on the fogger unit so you don't have to refill the small bottle it comes with all the time. Also use filter water to help keep the unit from building up deposits from the water. Having this installed will monitor and aid humidity all day so you do not have to constantly mist or worry about boosting the humidity. Both products worked really well for my and my BRB.
http://www.amazon.com/Zoo-Med-Reptil...ds=reptifogger
http://www.amazon.com/Zoo-Med-HygroT...5EXTPWA6KR68AY
I have glass tanks, pvc cages, and a rack. I don't find any of them any different when it comes to keeping them clean. In case you change your mind I bought my GTP cage from pvc cages and it is a great cage. You can add and they install perches, lights, rhps, and other options. Its top notch. You can even have them shipped assembled, which is what I did. Its my favorite cage.
http://pvccages.com/
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