Too cold for comfort!

[craigafrechette]

I wrap my glass enclosures in 1/2" foam board insulation. I do the back, sides and top. This helps immensely with heat and humidity retention.

Also, it's important to understand that Under Tank Heaters (UTH or heat mat) are made to go Under the tank. They are designed just for a hot spot and not for ambient temps.
Think of it this way...the UTH on the side heats the side glass. And since heat rises, that heat just goes up and out. So unfortunately that UTH is basically useless on the side. BPs definitely don't hang out stuck to the side glass.

You may need to get a higher wattage bulb or CHE.

Also, extremely important....ARE YOUR HEAT SOURCES REGULATED BY A THERMOSTAT???

A RHP, like mentioned above, is a great option, but a bit pricey especially for a temporary 20 gallon tank. Might be worth looking into for your next enclosure though.