Good morning,
We used a thing purchased from a medical specialty supply place that slipped over the arm and then a pump was used to remove all the air. It was very tight, and it dd seem to keep my daughter's cast dry through several days of swimming. However, the last time we used it, water did get inside somehow. Not sure if I failed to get it airtight to start with or if my little one pulled at it, but the cast had to come off. Fortunately it was supposed to come off soon anyway, so the doc did not replace it.
The thing we used was very hard to get on and off. Hope you figure something out. It sure is hard to keep the little ones out of the water.
Cheryl
Kenton, TN
21200AG sand filter