If you want a door you need some sort of frame and jamb, you can get real fancy and customize that however you want.

My suggestion would be to get a bifold door kit, you can cut three inches off top and bottom of the door a lot easier with a bifold than with a swing door. With a hollow core door when you cut it for height you end up with a hollow cavity at the cut and have to fill it in. With bifold doors you have to use the cut off pieces as a guide to drill similar holes in the new fill pieces. The door comes with a kit and instructions, you can install it directly in a drywall opening without any demo, the kit allows a bit of flexibility for adjustment for getting it right for height and plumb.

There are lots of other options, but this is the cheapest and most straightforward in my eyes. The main things you have to watch out for is that the holes in top and bottom get drilled in the right location. The depth doesn’t matter and they don’t have to be perfectly plumb, but if they aren’t laid out properly the door won’t swing right. Being smart about using the cut off as a guide or jig, and double checking before drilling is best bet. If you mess up you can re-drill the holes or replace the piece but it’s a bit of a pain.