I would make two changes:
- Use PVC for the shelf as it makes clean-up easier. I would make extra shelves so if one were truly foul it could be swapped for a clean one and then the dirty one would be soaked/cleaned in the bathtub and then sanitized.
- Cut off a shelf corner or just make the shelf a few inches shorter than the tub length rather than making an access hole. Snakes have gotten stuck in holes in décor by going partway through and then turning and trying to come back out, so their head and tail is on one side with their body stuck. Plus it would use a bit less material per shelf so you may be able to get more out of a sheet.