It wouldn't hurt to try some marine grade silicone sealant (boat store, Lowes, Home D, wallyworld) on the inside. Putting something on the outside is likely to fail because the pressure may blow it loose. If it's inside the pressure trys to force INTO the hole. Just be sure to clean the surface thoroughly before you apply the sealant, then let it fully cure before you reassemble the filter.

Hope this helps!

Pappy