Where do I insert the exit tile in a 1-2 player game
From the Rulebook


Setup

You will need the 6 main chambers and the 13 basic chambers (that is, all the chambers without the purple mask and/or treasure icon). Set aside the starting chamber and exit. Mix the remaining chamber tiles face down, then stack them as a draw pile. Place the starting chamber in the center of the playing area, then place 2 randomly drawn chamber tiles next to it, as shown in the illustration below. Finally, take the top 4 tiles of the draw pile, shuffle them together with the exit tile, then place them on the bottom of the draw pile.

Exception: If you are playing alone or with a second player, insert the exit tile in the middle of the draw pile.

Considering the rather loose and light nature of the game. I don't expect that the instruction was meant to be literal. But if you need a ruling I would remove half the pile. Place the Exit Tile and then call that done. So with 15 tile. Remove 8 tiles (7.5 rounded up) place the exit tile and then now you have two piles. 8 regular and 8 including the exit.