Hi,
I just built a very simple and cheap volume pedal. You just need a slider, a stereo panel Jack and some pieces of aluminium.
I use it for a midi controller, if you want to use it as passive volume control I suggest a good quality slider (e.g. alps).
To bend the aluminium I use a 70 Euro bender. I really suggest to buy one if you're into DIY - building my own cases saved me a lot of money.
If you're interested you can find the video I made on YT:
(https://i.postimg.cc/R6ywDbY5/schema.jpg) (https://postimg.cc/R6ywDbY5)
The volume pedal is at the bottom of the picture. The foot-switch is the midi controller I'm building...
(https://i.postimg.cc/H8b8PW5X/IMG-20201109-144120.jpg) (https://postimg.cc/H8b8PW5X)
Enjoy ;)