Setting up a large glass enclosure in a room where the ambient temperatures are low is going to be a challenge. Find something to replace the nubs. Anything that will raise your enclosure a comparable distance will do. Reposition the heat mat to the bottom of one side of the enclosure and set your thermostat so that it can maintain the surface temp in the enclosure at 27C. Position your heat lamp over the opposite side of the enclosure. Replace the bulb with a ceramic heat emitter. Set your other thermostat to maintain a basking spot of 32C. You'll likely need to insulate the sides of the enclosure. Foam poster boards might work. You'll also likely need to cover the top to keep the heat and humidity from escaping. A lot of people use tin foil or foil heat tape. All that, or you could save yourself the frustration and buy a pvc enclosure and a radiant heat panel.