OK here's what I would do:

Take the 20 long and turn it up vertically on it's side. Now the animal has all kinds of climbing room. Depending on what kind of mesh screen you have you can cover what would become the top 2/3 of it with Glad Press n' Seal to trap some humidity. Put a terra cotta clay dish full of wet coco fiber in the bottom. Under what is now the bottom place a 6X8" UTH to warm the clay/coco and generate humidity. I don't think a thermostat would be necessary in this case because the animal can't come anywhere near the heat source. With the plastic wrap it should trap a little heat as well. Fill the cage with plenty of branches, vines, and leafy things. There is a company that makes a great product called "MAGNATURALS" (google it)and they have magnetic platforms that will hold 2 little 1oz. plastic cups to hold water and food. Also others in many sizes for climbing on. They use the little cups you buy at liquor stores or party stores for jello shots. Mist the cage daily and change the food every other day. This would make for a great display cage especially if you turn the cage to view through the bottom or side. Well that's my two cents. Post some pics if you set something up like this.