How do I replace a non-standard sized AC closet door?
I have done this before 28 x 54. Your door width is a standard size your 'custom' issue will be the height of the door. Return air duct at the floor with no baseboard .. odd .

Anyway you purchase a prefab indoor door unit and you cut it down to size where there is no paneling design inlays. 

Mine just happened to work out nicely - these doors are typically hollow and have a 4 board outer frame - the thickness of the door - foam might be in certain parts but not all parts.

So After cutting mine in the correct spot -  simply separate the base frame of the bottom of the door (it is like 1 x 1 x door width) put glue in the hollow area of the door and on the 1 x 1 x door width piece - you slide that into the space and clamp it down between some 1 x 3's  - let it sit overnight to seal. 

Now you have your door and you will need to make the frame or modify the frame you bought with your new door (that is what I did) and the return vent sizing on your wall to fit everything nicely. 

So I made my return air vent and the mounting and layout and then installed my door frame - I knew the door size I had and measured everything and drew it all out on paper before I did anything. There is a 2 x 4 at the base of my door frame that goes across and the frame sits on it.

Below is the finished project - I did this about a year ago so I don't have the before pictures. I purchased my interior door and frame from Surplus Warehouse it cost me about $60.00 all inclusive.