I'm going to reply on RC just in case anyone else is interested in this or
not. A friend mentioned that a product used to keep plants moist is what is
in the cold water boots. I bought some, the one I bought is called Soil
Moist. I got it at Home Depot, and it is a granule or crystal like form,
when you add water it really swells, so if you fill something with them allow
plenty of room for expansion. They do have a website, like who doesn't, it
is www.soilmoist.com I think there was another brand, but this was an
experiment so I bought the cheapest!