Well, just for giggles, how about aiming, size-wise, to fit the PCB into an Apple external 3.5" floppy drive case? Dead drives could be updated with the new SD drive. And it's about the same size as a small project box, so one of them could be used instead where the genuine article is not available.
Hm. Speaking of which, various people on the other thread were requesting a version for the Powerbook 100 and Duo floppy port, which uses a different connector. Others mentioned internal mounting - different connector again. Is it worth designing the board such that all three connection types can be accommodated? I would be happy to help chase up dimensions etc if you wish.