Check the local fabric shops and see if they have eco-felt. If you can sew, or even with just a glue gun there's a how to over in the DIY stuff somewhere to make your own. Could probably make a few small, half gallon size with a yard or two of fabric.
I got fancy on a batch I made up and even added in some Velcro to use them as starter pots easy to transplant out of for a pepper plant project I was going to do.
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