Cut/make a cardboard box near the size of your tank and then make/cut/fold another box that will slide into the first one. No bottom in either box. Then you can telescope it under the tank and tape it.
First have a 40 watt bulb encased in one of those wire protectors inside on a brick. That should keep the tank from freezing.
When traveling, remove tape, separate boxes and fold flat for transporting and you can use it next time. Telescoping will allow you to adjust for different terrain.