I sorta-kinda sew. That is, I can use a needle and thread to sew a decent, if unlovely seam. And while my mother sewed sheets and curtains that way (as a child! Of four!) I'm not up to that level of snuff.

If I were going to make my own "custom" curtains for that bed, therefore, I'd buy a pair of cute twin sheets and staple them (folded over as necessary) to the inside of the bunk with a staple gun.

I like the space theme, because you can get those cool glow-in-the-dark solar system sets to glue to the ceiling!