Wombat, you're on the slippery slope. There;s a law of nature to the effect that there are no inexpensive wide angle lenses for formats larger than 35 mm still.

Suggest you visit www.schneideroptics.com, use the info tab to find their data on older lenses, and look at their short lenses' flange-focal distances. Then redesign y'r box (shorter! That's the Alpa trick) to allow you to use one of them, 47 SA, for example.

You're falling into a trap I'm familiar with. Have a cute idea based on a very inexpensive piece of found equipment, start thinking about what more can be done, and get fixated on using the piece of found equipment instead of thinking about what doing the what more really requires. Been there, done that, don't recommend it. Open your little marsupial mind.