I've used a lamp or che over a tub many times for temporary setups.

I always used a lamp stand to keep a safe offset from the plastic. Then you can use a tstat, dimmer, or distance to control the heat delivered.

I have had good luck with distance but I have steady temps in my home so that works for me. If your temps will have much fluctuation I recommend a tstat or dimmer at least.

With the ZooMed lamp stand you can raise and lower the fixture as needed. Of course brining the fixture closer delivers more heat while moving the fixture farther away reduced it.

The lamp stand are $20 at both big box pet stores in a yellow box.