1. Get a styrofoam box or cardboard box and line it with styrofoam. Place a piece of glass on the styro and put heat mat on it then the tupperware. I've used this method for my feeders and it works great. You can cut a window to allow light in.
2. If you have an old snake cage or wooden box you can do the same. You don't need the glass on the bottom. I'm using this method now and it also works great. Another thing I'm doing is using an old gecko cage with a ceramic bulb and computer fan to circulate the air. The ceramic is on a dimmer so I can control the temp. If you're going to do this get the more expensive sliding dimmers and not the cheap dial/rotary ones. The dial ones can often shut off after a blackout (after the power comes on), or if turned down less than half way they can also be unreliable.
If you have concerns about too much heat you can raise the container above the heat pad using wood spacers. I've raised my baby emps from 2I to 7I with this method and so far so good.
Just have a good thermometer in so you have an idea about temps and have the heat pad on 1/2 the container, or have one end raised higher than the other to give a temperature gradient.
Hope this helps!