House rules to make the cloister less of a game winning tile in Carcassonne?
There are a couple of expansions that reduce the usefulness of cloisters. You don't get the points until all the surrounding squares are filled, which is a safe bet in 'vanilla' Carcassonne, but not such a sure thing with expansions. The Tower expansion makes it less attractive to leave meeple on the board for any length of time, because they are sitting ducks for capture by a tower. The Catapult expansion makes it possible to remove Meeple from the board, or replace them with your own. The Princess and the Dragon expansion also encourages Monk removal as dragon-food.

You could ban the 'instant cloister' case, disallowing the dropping of a cloister into a hole for an immediate 9 points. I personally don't find this a huge problem though. You have equivalent 'instant city' and 'instant road' cases. Ultimately, there is quite a lot of luck involved in Carcassonne, by design, I think.