The only thing I can think of is using duct tape on the OUTSIDE of the tank around the hole. Silicone would seal it but then everytime you need to get in there you'd have to peel it apart and reseal. Duct tape's quicker!
Someone else may have better ideas though. I've never used a filtered water feature in my tank. Just a good old plastic tub.

Just make sure if you use duct tape you check that all sticky parts are well away from possible snake contact!