Every water feature I have had in a cage was removable. Have you thought about cleaning this pond out?
Snakes love going to the bathroom it their water and having a built in pond it going to make it very difficult to empty and clean.
If it were me I would get a good size water bowl and place it in the pond area, then I would add a lot of substrate around it to make it an in ground pool. This way you can take it out and clean it if need be and you will still have a good size water source. Plus the deep substrate around the bowl would be prefect to plant more plants in.