I went with MDF, since it tends to be A LOT staighter than plywood, with less bow. I had them cut them to size, which was 1/2" bigger than the opening, so they overlap the face of the stand.
I used the same trim thats around the bottom of the stand, to frame the edges of the MDF pieces.
I then cut them to size, using the miter saw, at the 45 deg. setting.
Using small 3/4" wood screws (and the help of clamps), I screwed the trim to the MDF, from the the back side.
Next is to mount them to the stand, using "flush" style cabinet hinges!!