patching

I'm installing all new instruments and will be putting new holes in the bottom for the new speed and depth transponders. I don't know what to do with the old holes. I suppose I could just leave the old transponders in there, but I'm trying to keep the bottom as smooth as possible. Any ideas...

Hello, All. My Ericson 35 Mk III cooler has a hole in its side (about 2" round), that shows the fiberglass behind it. I would like to patch this so that water can't get back there. I'm not sure if the liner is made of. Any suggestions on a nice, easy way to seal it? Thanx for any...