Personally, I wouldn't use duct tape. Go to your building supply store and have them cut out a piece of acrylic that you can just lay on top or use aluminum foil. You want to make sure that your BP doesn't accidentally get stuck to whatever you're putting on top.

In regards to the T-stat, good ones will be pricey. If I were you, I'd look for a used one. I found both of mine Herpstat 4s for half the price online. Similar brands to Herpstat are about the same price. For the Thermometer/Hygrometer combo, you can pick one up at Walmart for like $10 bucks and just place it inside the enclosure.