I saw an article on here about using a baby diaper to keep moisture

https://www.hometalk.com/diy/grow/plants/using-diapers-insides-for-gardening-8335855?r=2

Yes they are filled with a water retaining gel, so would be great.

Here is one, hope it helps:

https://www.hometalk.com/diy/grow/put-your-baby-plants-in-diapers-8004796?r=2

Do a price comparison to a package of diapers to a jar of soil moist.

Heh, I looked it up and saw the link I was going to share is the exact same as Dysko's. (You beat me too it!) ;)

Place an unused disposable diaper absorbent lining facing up, in the bottom of the plant container. Put in the soil and the plant. The diaper keeps the plant moist and the watering will not flow through it.

You can buy Miracle Grow watering crystals to put in your plants and even your yard plants...$10 a bag goes a long way. We love these!