For a number of reasons I'm considering a translucent plastic storage container - the best variety of size options, lightweight, easy to carry and clean etc.
I see that this style is suggested in the Habitat page of this site:

But to me this deep-style box looks a bit too much like a temporary container to store old tools etc. in a cupboard.
So I've looked around and have found different styles. I'm looking at either a shallower square box like this:

or a wide clothes-drawer style container, which almost looks like a "real" tank:

I imagine it's best to put the box on a layer of foam to distribute the weight evenly so it doesn't crack.
However I'm concerned that if I put my heater in these boxes it could be dangerous, either by melting the container or by heating the plastic which might then leak some chemical into the water. The heater has a plastic shield around it, but I'm worried that over time the heat could still cause problems.
Are these legitimate concerns, or is everything totally safe?
Thanks in advance for any advice.

