The only technique I know of is to spray with triple thick and then lay Saran Wrap on top while its still wet. Then with your hand or fingers press the artwork down onto the glass. A rubber roller would probably be best but I've never done that. Let everything dry overnight and then remove the Saran Wrap. Make sure its completely dry before removing the wrap. Its amazing how the wrap does not dry to the glass.
You will have some cracks in the artwork but its much better than losing it all together. And sometimes it looks fairly decent and can be touched up with paint over the triple thick.
Note: I have done this many times but *never* with large pieces. Be careful with the triple thick propellant. You don't want to blow any chips of artwork away.
I'm nervous of the advice I've provided here because you say the pieces are large. Hopefully someone else will chime in.