I would second the idea of just simply forcing threads into the plastic. It actually does work pretty well, assuming the plastic is thick enough to get at least a couple turns of threads going.
Another line to change, though? Are you letting the silicone set up for very long before adding water? Silicone is one that does need air to cure well and with it closed in, it can take what seems like a long time to cure. When pressed for a cure, I use silicone on some pipe joints that I know will have to come apart later. However, I have learned it is safer to wait at least 6-8 hours to make sure things will hold before I start using them. So maybe an alternate might work if you give it more time to set really well??
Also I have found that small parts can be sealed with super glue when there is a defect that won't seal otherwise and it is low pressure like this. If you can get the hole cleaned well, maybe a quick try with super glue would be a worthwhile shot?
More than one way to skin a cat, so finding the right one is the trick? Good luck!