What do you guys think is the best way to re-attach this to the sill plate? I was thinking of trying spray glue, but not sure if it would last long-term. Maybe epoxy?
Several spray applications of high strength contact adhesive IMO.
Let Dry in between coats
Self tapping screw?? 
Would a thin piece of automotive double sided tape make it sit up to high? If so, my vote would be the epoxy. Good Luck!
3M spray adhesive. If possible, use the high strength version.
Isn’t there an adhesive sticker on the back of the emblem already?
The adhesive that comes with these gives up after a couple years. I reattach them with RTV. Just a couple dots of it will do.
Weatherstrip adhesive?
I’ve done what Royce suggests with good results as well.
Ended up using some 5 minute epoxy I found in the garage, seems to be holding well so far. If they come loose again I’ll give RTV a try. Thanks for the help guys.