This is pretty great. A little mad I never came up with this one.
Using the space where the bearing used to be is a great choice. The bearing was removed in the 2nd year because it was causing some misalignment issues since that bearing would have been the third bearing in the VP assembly including the two in output section.
Added
Any reason you couldn't use the 30 tooth gears in the VP dual input stage adapter? They have an OD of right at 1 in and spin freely in that space. The teeth are a bit smaller but I don't see anything in the sensor spec sheet to prevent it from working. You would also get a bit more resolution by going up to 30 teeth.
I think you could make 3 of them form this $10 kit.
Also were you running the sensor at 12v or 5v?