eternal entrapment

I couldn't find a "short story" category, so Culture: Book: Subject will have to do. Here's the idea:

The main character wakes up from a "normal" reality to find himself caught in some existence where he is stuck in a dimly lit grimey concrete box half the size of his body, with his arms tied to his legs behind his back. The box is floating through interstellar space, billions of light years from the nearest star. He is never physically comfortable, always trying to straighten his body out and catch a full breath, and he can no longer sleep. And he is immortal.

The story could go through all the stages of his thought processes, perhaps through eons - Initial waking, then realizing his situation, then panicking, then relaxing and thinking about it, then realizing that he hasn't slept for 3 days and can't sleep and panicking again, then realizing he hasn't died after a month, then trying the scratch his way out of the box using his shoulder after 2 months, then trying to trance his way out after 6 months, then going crazy after a year, then creating his own music and languages and games after a couple years, and beginning to create his own mythology and universe after 10 years, and revisiting him after 100 years, 1000 years, until his mythology actually becomes a reality, a new universe - he is the God of a new universe