Cutting the slot is really not a big deal. Except that you're painted already and probably don't want to back track.
You can buy plastic or wood laminate edging that irons on for use with melamine stock for shelves and such. You should be able to find black, white and wood (usually oak) at any decent lumber supply. Home depot has it for sure. Don't know if it will work over paint.
I think you'll get a secon rate finish by cutting the spine out of t-molding. When you hammer t-molding into the slot it flatens out against the edge giving it that smooth look. It's not designed to be glued on, and think that you'll be fighting it's springy curved section the whole time you're trying to glue it.