Welcome to RC Universe!

Now, onto your question.

You can either go to your LHS and buy a new fuel tank stopper for a couple dollars, or block it up.

You can either block it up or just use that third line and let it hang from the bottom of the cowl with a fuel line plug. Makes it easy to fuel up, but it looks like you've already got that figured out.

I think I would just stick some leftover brass tube in there. Kink it on the ouside of the tank and then put some fuel tubing over it with a plug on the end. It should be fine...

As far as the epoxy in the fuel, I'm not too sure. See if you can knock it out of the hole before putting it all back. If it's breaking up in pieces, it is going to clog up your carburator. Are you using a fuel filter? Might be a good idea for a while...

How do you like that Cap? I loved mine. I had a Pro .46 on it and it was perfect! I miss that thing...