Gags, try making up a thick paste from good natural attractors such as Pre-digested fishmeals, belachan, yeast, GLM powders and so on, and plug the hole with that.
This tactic also works very well with drilled wooden balls as hookbaits, when fishing a lake with crays.
I will say my idea of the wood was inspired by Newmarkets sponge in a swim feeder. The reason I recommended wood, is that a lot of sponges are made of plastic and so aren't particularly environmentally friendly. There are biodegradable and natural sponges out there though.