The original design did not even use a pot. I'll stand by my first comments since I know how this will effect some devices that might be plugged into it.
If you can't find the panasonic pots, try the clarostat 585 series. They are available at Newark Electronics ( www.newark.com ) as part number 585DX4Q25F103ZP at $2.48 each. No waiting. Or just build the original which did not add a resistance path to ground ahead of the input cap. In this case, you rely on the volume control of whatever is plugged into it.
Part of your parts availability problem is due to the pot having an on/off switch. Another alternative is to purchase an audio taper pot without the on/off switch and use an input jack with an additional leg that sits against the ground barrel of your cable. Attach the ground of your battery to the additional audio input lug which is only grounded when you have something plugged into the unit. This will also keep you from losing batteries if you accidentally forget to turn it off when you disconnect. (Example below).
This is easiest if you are using a mono input. You can use a stereo jack. However, if you will be connecting two of these circuits to a stereo jack, you can still get jacks with additional connections in them. They are just not as common.
MP
View attachment 39893