Maybe you've seen those pots in the stores that look very heavy but are in fact very easy to lift? I think this is a great idea, however their prices are still unreasonably high.
Here's one way to make a large lightweight pot that could pass for being made of concrete on casual inspection.
Step 1: why bother?
I wanted to plant some things and went to get my favorite large pots from the backyard, but look what happened to them! I pondered how to fix them and decided I would just try to make something new.
Well sure, it's a lot less effort to just go buy a pot, but I always wondered if I could mold things out of spray foam insulation, and after a few tries, I found out that I could.
It remains to be seen how long it will hold up to the elements and whether plants will tolerate being in a pot made of polyurethane foam.