I'm with Jada, heaters are fine for plastic containers. :) They don't get too hot to melt the plastic. :)
I had one who lived in a 5g filtered water jug for awhile until I was able to get a 20 long for him. :) I just cut the top off so I could get into and out of it.
I also realized that 1/2 gallon plastic water jugs are awesome QT containers. Since they're square, you can line them up in a larger tank and easily heat several, and allow females for QT to see one another before they can actually get to each other. Also with the 1/2g size, water can be changed daily and you'll still have low ammonia. Anything smaller will have higher ammonia quicker than once daily. :)