Try aluminum pans or plastic bowls from $1 store, or some other kind of cooking ware.

I have also seen people using styrofoam lids or containers that might fit. Another idea might be some kind of plastic that goes around the outside, then tie it up to the bag to keep it from falling, but also need to be cautious about the heat build-up.

Thanks so much!!! I knew it couldn't be that complicated, but I had not seen this done anywhere, nor did I think about it before I started planting the pots on the balcony! Thanks again! I'll try the dollar stores too.