You can turn any such maze into one with a single exit by surrounding it with a small “moat” with a single exit. For large mazes, I don’t think the price you pay for it is too large, as it only adds O(\√#cells) of area to the maze, and I think any good measure for complexity of a maze scales by #cells (if you don’t scale by #cells, you can make a maze harder by glueing multiple ones together, and the trivial maze where there only one straight line will get harder by making it longer)