First off, Welcome to the forum.. Yes you can use a UTH with a plastic tub. You need to have a themrostat in there to control the heat so it does not overheat. To put a heat lamp or CHE in a tub you will need hardware cloth.. Just cut out a square big enough so none of the lamp can touch the plastic and staple the cloth in place... You can also use hot glue if it is far enough away from the lamp... Hope this helps,
-Matt