That is the hardest part!!!
A very important detail is to keep the top and bottom tracks parallel to each other so the doors will slide side to side and that the top grooves of the track line up exactly with the tracks in the bottom grooves so that the doors stand perpendicular to the track.
Also, plexiglass or whatever door material you will use needs to be smaller than the groove so that it will slide.
There will necessarily be two doors half the length of the opening, one in the front groove and one in the back, so that you can get in to the storage bin.