I think glue is your only hope. People have used hot glue guns, I assume a crazy glue would be good. The only other things I can think of I wouldn't do myself:
1) Nail/staple it in (that has to look ugly unoess you get nails the same color as the molding?)
2) If you could somehow wedge paper over the part that goes in the slot, then push it in - where the paper would thicken the part in the slot enough to keep it in there. Again, sounds hard to do without seeing the paper and doesn't sound too permanent.