I removed the whole plastic guide that was sitting at the bottom of the frame.
The pipe that guides the front derailleur came with it.

Now I have a ~6mm hole, just behind the bottom bracket.
It collects all the water/mud it can.

I use a duct tape for now, but need to change it.
And was wondering if you found a more permanent way to plug it.

I can't quite picture what the hole looks like -- could you send an image?

With limited knowledge, I would try using a rubber plug whittled down to size with a 'box cutter' razor blade...

I just used a dab of clear silicone caulk.

Joelk -- yes, silicon sounds a better choice...

I used black silicon. I also used it to cover the hole on the head tube I am not using.

ok, I'll get a black silicone tube and plug it.
I was wondering if anyone tried some epoxy or some other resin with carbon patches.