Thanks for all the advice guys! Where can I get the cabosil and microspheres you mention above and is there a certain ratio I should mix it together?
Microsperes make the epoxy easy to sand. The cabosil makes it thicker so it works better as a filler. You could use microspheres alone for this job. You will only need a little bit of resin to make enough filler to do such a small job.
You may want to use Marine Tex instead of making your own filler. It's a bit pricy compared to making your own filler but it is very simple to use. (Don't mix all of it. Marine tex is 5:1. You don't need to use all of the stuff at once.)
As far as the cabosil/microspheres, there is no exact mix, but a basic baseline is 50/50. You can mix it however you like to get the kind of filler you want. As long as you don't use too much of either and dry out the mix, it will be fine. You mostly are just mixing it to make the epoxy thick enough to use as a filler. You could use all cabosil but it will be harder to sand, and you can use all microspheres but it will be hard to get the consitancy you want. Just add a little of each until you get the consistancy you want (peanutbutter, or maybe a tad runnier). Just know that 1oz of resin mixed with filler is going to make 2-3 oz of filler. That's a whole lot for the job you plan to do.
The method I suggested is going to leave very little need for any filler. Actually, if you put extra resin in the hole with the glass, it may sit tight to the tape and you wont need any filler at all. Weight is of little concern on such a tiny repair.