In another thread, Bart shows his setup. It's even more foolproof than simple markings.
I (obviously) agree, but it only allows to correct in one direction (which it does very well). It offers a small package for light travel if one occasionally could use the additional field of view but doesn't want to use a full pano setup all the time. Of course one could add a similar construction for the orthogonal direction, but that would still not cover the other possible rotations.
I have some reservations, besides its price, with regards to the Hartblei solution or similar ones. I'm not sure how well the lens
will support the heavier DSLR bodies, such as my 1Ds3. And of course, the final image will only be as good as the resolution of the edge of the image circle. The TS-E 24mm 3.5 II is quite good, but requires stopping down to f/11 or f/16 to increase in the edge quality, while losing center resolution to diffraction at the same time.
Here is an image that shows the type of reduced quality near the edges, using the single (shift camera right and left opposite to the lens) bar and stops:
Click on the image for a full resolution (11MB !) version
IMHO, much higher quality can be achieved by using the center of the image circle with a rotational stitching method. It also allows to use the lens at f/4 - f/5.6 which offers much faster shutter speeds (may be necessary to avoid vibrations, e.g. when shooting from a bridge with heavy traffic), and even higher resolution. It also allows more creative use of the tilt functionality.