I would suggest either hot glue or contact glue (hot glue keeps it in place, but is easier to remove if need be). T-Molding.com also has some molding with a larger neck (I had a similar issue to you, but fortunately I had ordered a bunch of samples and went with the once that fit snugly). If you want to get molding that fits properly, I'd suggest emailing them with the dimensions of the slot you need (probably ~1mm larger than the molding you currently have, but that is just a guess), they're very helpful.