Installing bifold door at top of stairs

I want to install a 36" bifold door at the top of a stairway going to the basement. I am dealing with plaster walls. The ceiling is on a pitch (the degree I have not checked yet). On either side of the opening are doorways with casing leading to other rooms. I have just enough width of floor space (about 2") where the door can go. Wall to wall is 38" and between the two casings for the other doors is about 36" I hope this is clear the area I am dealing with. My first question is how to attach the top guide rail since I am working with a sloped ceiling. Do I need to rip a 2x4 with a bevel to give me a flat surface to attach the track to?? What other options are there? Since the casings for the other two doors are so close to where I want the door, will there be enough clearance for the bifold to open and close and not get bound up on the other casings?? If there are other options to provide some sort of closure to the top of the stairs, I would welcome other ideas short of putting up one of those ugly accordian folding curtains, which is what I just took out.

Many Thanks in advance for any assistance.