I'm just guessing but it sounded like Tinker was looking for a quick repair idea with minimal time, tools, and expertise required.  That being said, without a doubt a plastic weld would be the gold standard repair method.

I got the impression Tinker wanted a fix that would look clean and not "repaired".  A small 1/4" hole or smaller could easily be welded to look natural. A 1" hole, not so easily.

Yes, yes, you're both right.  I'd like to not make a career of it and I want it to look good for the grandkid, but all of the suggestions have been for good ways to seal a large-ish hole that's no longer needed.  If I don't use some of them, someone else will surely benefit from what's been suggested, so it's all good stuff.
 

Thanks for asking.  It's not finished yet - other things keep getting in the way.  I've welded a 2" disc of scrap material (from Wilderness Systems) from the bottom/inside to ensure a proper/permanent seal.  I haven't cut, shaped or installed the "plate" on the top side for good looks.