I had basically the same situation in my 70's-built house, the closet doors went all the way to the roof, and were horrible looking bi-folds.
Basically what I did is framed in a header to bring the opening to 81" (remember to account for 1/2" drywall while framing):
Then some drywall and paint (note I didn't yet repaint the ceiling in this picture):
I had 4 closets like this. One by the front door got sliding mirror doors, the rest got frosted-glass sliding doors. The right picture below is actually the "after" shot of the first one in this answer.
The width is going to be challenging for stock stuff. If it was a bit wider, you could probably split the door into two by putting a small column in the middle, but that would leave you with a couple 3.5' doors, which are pretty small. If you went to bi-folds, you might be able to do a 4' and a 2' door, though that might look a bit strange.
A quick search turns up some custom closet door manufacturers, which might be an option to get multi-panel sliding doors, like this:
If you do this you can probably also get them 8' high, but honestly, even without the doors on, just making the opening 81" really updated the look of the house.
Yet another option is that you could shrink the width down to 72" (basically just extend the wall/framing on one side by 9"), and along with making the height 81", you're into a standard size where you can get both sliding or bi-fold doors off the shelf from any box store.
Good luck, and let us know how it turns out
If you just want the bathroom door held closed, remove the latch mechanism totally, install a dummy knob and use a magnetic closure to hold the door in place.
You do not even need the magnet to touch the matched plate on the stronger ones. If you are using a bit of foam or a vinyl bumper on the door itself, adjust the magnet back from the plate until it is about 1/16 inch away. It will hold without a click.
If you need a lock for other times, use a surface mounted privacy bolt.
Best Answer
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.