Re: wall for 2nd bedroom
I think your options really depend on the width and height of that door opening. Some standard openings have a lot of options. Custom sized openings, not so many and/or very expensive.
If the opening is a fairly common size; such as 26, 32, 34 or 38 inches wide and 82 inches tall, you can pick up a prehung door for about 66 dollars and, adding a few dollars for trim, have a real door there. For another 40 dollars you could get a quiet solid core door. That's if you feel like installing a door.
A better, maybe cheaper, choice for an apartment would be to install an exterior pocket door. Essentially, this is a slab door (solid or not), hung on a sliding door track which is mounted externally on the wall under a ledger (piece of 1x2 attached to a piece of 1x2 to make an "L") and the sliding door track is attached to the ledger bottom. There are many ways to do this and the success all of them hinge on who's doing it and they're experience and/or creativity.
When you move, pull the screws, patch the holes and off ya go.
If I only knew what I was doing!