OK, I got it. Fully understand and I will repair the hole this week. Thanks everyone again for your advice & help and thanks Scottyt for the videos. Somehow I missed them when I read the responses. Sorry. You're a great bunch of supporters and novices like me really depend on you. I'll post some pictures when completed.
What will likely be surprising is the number of layers that it will take to build up to 3/8 thickness.
Also I'm not sure I would attempt to do all 3/8" in one big bloop. If you are working upside-down which I suspect you are it will want to droop out.
I would probably do a patch with maybe 5, 6 layers and apply it and let it kick, making sure it is stuck up there. Then do some more layers and stick them up. Keep going until it looks good. Then hold a piece of plastic and squeegee etc.
If epoxy gets too thick it will actually get hot and start to smoke if it kicks all at once.
If others have done 3/8" in one big patch with no problems either drooping out or getting too hot I'm sure you will get some first hand accounts.
If it is you first experience with epoxy as long as you put the second coat on while the first coat is still tacky you will still get the good chemical bond.
And it may be a little easier to see how it is coming along.
Another point is that you want the glass just whetted out. The goal is to use as little epoxy as possible without leaving any dry glass.
Also thickening the epoxy with colloidal silica may be necessary to make it sticky enough to hang upside down.