Re: Filling holes
First thing that comes to mind would be to use a section of a seat post. It should slide down into the tube and cover the hole you make. Then, being very careful (you'll need some welding experience here) weld the post, where it shows, to the tube. This will do two things: Fill the hole but also reinforce the damaged area.
Good luck, and good luck with that HF welder.
EDIT: I've thought about this a few times today and it occured to me that there should not be a hole when you remove the girl's bike downtube. It is probably welded to the outside of the seat tube and when you cut it off it will not leave a hole but only the metal and weld bead.
A grinder should take care of that and you shouldn't be faced with filling anything.
Just be sure that you replace it with a top tube that will support the weight of you and the bike and is structurally strong enough. We've seen builders remove the top tube leaving nothing to keep the bike from folding in half but the down tube and bottom bracket. That won't work.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"
Last edited by 2door; 11-29-2015 at 07:04 PM.