leaking fountain - sealing stopper?

I have a patio fountain that came with a large rubber stopper(2.5 dia x 1.25" thick) and a short roll of green colored putty on a roll. The stopper is tapered, a hole in the center and a slit for setting in the pump cord and light kit wire.
I cant seal this stopper well enough in the basin to prevent the fountain from leaking. In additon to using the small amount of rolled putty I have tried to use silicon which has worked the best so far but still leaks(very little). So after several attempts with the putty and silicon I havent been able to get a perfect seal. I want to be able to remove the plug when I need to drain the water - so it cant be a permanent fix.
Most silicon tubes do not recommend for underwater use, so is my current silicon seal short lived?

Seems like an easy thing to do but...ideas/suggestions??
- should I use another type of putty. Plumbers putty?
- butyl rubber for sealing(which I can get in a tube @ Ace Hardware)?
- a better technique for using the roll putty?
- aquarium/marine sealant(but reads its meant for glass)?

thanks for your help...
Chris

In the electrical department of a hardware store, you will find a block of heavy putty which officially they call "duct seal compound" but unofficially everyone calls it "Monkey S%*T". This will fix it.

Bulkhead fitting with a dollop of silicone¿

I like the idea of a bulkhead fitting but think I will still be fighting to keep a seal for the pump and light kit cord. But thinking a Heyco type connector might work.
I will try the duct seal compound first and see what happens.

thanks for the replies...
Chris