Alright, so the glue gun didn't work as I had hoped for. Instead, (and I figured since the frame is more than an inch in thickness) I could use tiny small nails and just hammer them down onto the back side of the frame:
Luckily, after I had
tweeted my project,
Iamdavidbraun was smarter than me and thought of using fishing line. (Why didn't I think of that?) The line worked perfectly. I was able to tie a tight knot around the nails to hold a firm line across the frame to hold more than one pair of earrings.
Good thing my mother wears small dangling earrings and not those GIANT ones that drag your earlobe to your neck ... they look pretty but not when it is deforming your lobe.
I measured the gap between the nails (2 ½ inches):
And voilà!
My gift is done! (click on image for a close-up)
I hope she loves it!