There are several ways to do the input, indeed one has to be careful to protect the axoloti from damage. The euxoloti design uses external ADCs as extra protection (they are cheaper to replace than the axoloti), but the analog circuit from there can be used also directly. Here is a slightly modified version:
I changed it to use the -12V reference voltage from eurorack. It has the transfer function:
Vout = - (33/100)Vin + (33/240)12V
Note that it is inverting! But that is easy to correct in the axoloti patch.
Important point is that it relies on the opamp to clip the voltage to 0-3.3V, and it needs a special "rail-to-rail" opamp which is powered from 0V and 3.3V. To use TL072 a different circuit is needed, maybe I get around to posting an idea later.
I added R9 to limit the current that can flow into/out of the axoloti pin in case something goes wrong with the voltage clipping. This then uses the internal protection diodes of the axoloti chip. The datasheet says you can put 50kOhm max without reducing the accuracy.