how about a hotshoe spirit level like this? http://jefflynchdev.files.wordpress.com/2008/11/bubble_level_blog.jpg?w=471&h=571 since there are 2 hotshoes mount.
just a little comment on images. While it is great to be able to look at the "wideness", the cropped mountain top kinda make me want to "pull the image down" to view the top of the mountain as well. Maybe a interchangeable lens that can capture a even wider and "taller" image? I am probably commenting on something I am not familiar with. Apologies for that.
With all that said, I still can't said enough how good the iq of the images are.
I figure that at 1:4 pano ratio, my option of composition is more limited. But I look at this as a pros rather than cons as it forces me to work harder on composition. Also this project camera is all about low cost entry into the world of pano film photography, hence I avoided the complexity of interchangeable lens system. But I feel that, at least personally, the attractiveness of pano format is that it gets my mind involved when looking at a picture, a bit like the "pull the image down" feeling that you have.