I'd also like to know what app that is.
I do think the tenon is too small when using aluminum. Seems like I've heard that an acrylic blank will flex a little when drilling and cutting. So all things being equal, the threads on something like alumilite or ebonite should be a little tighter than threads in aluminum. I think the 1st thing I'd do is do the piece with the tenon again and use 11.95 of maybe even slightly larger. It's true that your hole may be too large too, but I'd make one part, and see if things improve. If so, but you still think there is too much slop, then you can do the other part.