That Curtis Stellar is the obvious arrangement with two mirrors in series, everything close to being at right-angles. Easy to build without much calculation.
However if you're happy for it to be "crooked", you can do a couple of things:
- bring the film planes forward toward the lens and at an angle, which means that the mirrors are steeper, which means less optical path length consumed, which means you can have shorter lenses
- put in three mirrors, which gives you four outputs and therefore a TTL viewfinder!
Of the first point, consider the Red film plane of the Curtis above, swinging it around to lie immediately to the left of the lens. You can imagine that the first mirror is steeper, which means you can bring the second mirror closer. Getting equal path length (for equal focus) means that the red film will actually be a long way out the front, next to the lens.
Of the second point, consider three mirrors in a tree topology. First mirror splits light into two beams, each secondary beam hits a second mirror, which splits it into two further beams. Net result is four copies of the light from the lens: one for each colour and one for the viewfinder. Each beam of light has been through exactly 2 mirrors and so has lost (assuming 50% silvering) 2 stops. Given that you need two mirrors in series for a basic tricolour camera anyway, this design shouldn't have any added flange distance (it makes the scrunching process described above harder though) and means you get a viewfinder.