Do you already have the T-Molding? Looking at T-Molding.com, they have 13/16 inch and 1 inch sizes. That means you'd need either 1/16 inch Plexi (which is pretty thin), or 1/4 inch plexi to accomodate those sizes of T-Molding.
Alternatively, you could round the bottom of the 3/4 MDF so the height at the edges is 5/8 and then use 1/8 plexi and 3/4 T-Molding. IMHO, 1/8 plexi is the easiest to work with and 3/4 inch T-Molding has the most options (colours, etc).
Or another option is to just redo the panel with 5/8 MDF and use 1/8 plexi per the above.