New age of insane prices on tubes
If the bottle has to be the same size as a 12AU7, then I think 5687 is going to be the best choice. Unfortunately, it looks like prices on those tubes have gone a bit crazy. They used to be a $4 tube, now it's $15 and up.
Then again, I see people selling 5814 (industrial 12AU7) for stupid crazy prices these days. So maybe the 5687 is still something of a bargain at "only" $15 each.
Possible problem is that the 5687's pinout is different, and its heater takes double the current of a 12AU7.