Make the most nonsensical, surreal thing you can think of. Lots of sinister lighting, with a subtle tint of unnatural colours, but not so far from reality as to seem completely alien, just a bit off.
Have the main character exploring an environment of some sort (preferably somewhere enclosed), while unexplained events happen around him and things are not as they first seem. For example, a man could appear in a room with your main character, run to a window, close the curtains behind them, only to have disappeared when the main character investigates and the window is now a brick wall. The main character breaks through the wall to find a small tunnel. Exploring the tunnel he finds a dead end. Going back out the tunnel again, he finds himself in a different room to the one there previously, and there are various suspicious red stains leading to the only exit. After a long and tense build up, he opens the door to find a small broom cupboard with a microwave containing a rubber duck, sitting on a stool. And so on and so forth, in that fashion for about 40 minutes.
Perhaps you could make it relatively normal to start with and then slowly build the mad world around the main character as they lose their sanity and eventually culminate in the character snapping back to reality, having just inadvertently murdered someone, preferably one who you would establish early on as someone the main character cares greatly for, creating a nice contrast to the main character being the victim of their own hallucinations and having committed a terrible crime, destroying something innocent and very important to them.
Maybe include an epilogue with police questioning the character, and or going over the report as a means to give some explanation to the fatal visions. An added twist could be that the hallucinations are somehow the fault of the murdered character (the main character's partner might put LSD in his drink as a way to get him to drop his reserved and uptight nature and loosen up for a bit which of course backfires horribly, for example).
This would be fairly simple to do and wouldn't necessarily have to cost very much. Most of the atmosphere could be done with relatively simple visual editing and good use of sound/music. As a fairly slow paced idea for the most part, for the sake of tension, you would be able to fill the 40 minutes without actually having to work out a lot of story or dialogue heavy scenes.
You might want to go for something more mainstream and story oriented though.