i am also using 18mm mdf, with 3/4" from tmolding.com. I actually think it fits perfectly, i like the tiny amount of overhang.
You wont find tmolding that fits perfectly. Use the 3/4" tmolding and then simply trim it back so that its flush with the wood. Trimming can be done with a sharp chisel, razor blade, knife, router etc etc.
But remember no to trim it to size until AFTER you have painted/laminated/whatever, or it may end up too narrow due to the added thickness from painting/vinyl-ing etc.