over the years I have had a lot of conversations about what to use for water dishes. A Lot of people use a lot of different things, but what has worked best for me has always been 4" PVC caps that can be found at Lowes or Home Depot for a very reasonable price.
The 4" caps hold a 8 ounce deli cup perfectly. For hatchlings I use 1 1/2 x 1 1/4 reducer bushings.
The reducer bushing hold a 1 ounce sample cup perfectly. You can find the deli cups and sample cups at Smart and Final or similar stores.
I hope this quick tip helps someone out when trying to figure out what to use for water-dishes for their cornsnakes.