WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I would try the 6N1P-EV. It's cheap and very good. The EV version is long life. It was designed as a true low frequency voltage amplifier. It is 6.3V though with pin n.9 connected to a screen between the two halves. Amplification factor is 35 and average gm 4.5-5 ma/V at 150-180V/6-8 mA. Input capacitance is higher than 12AU7 though.
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.