And a follow up video with a more refined method:

Eight pressure-sensitive Velostat/Linqstat pads for a velocity-sensitive MIDI controller

There is also this method using piezo sensors, but from experience I know that this is completely insensitive to sustained holds. It’s used for electronic drumkits because it measures percussion, not pressure:

DIY midi controller with 8 Velocity-Sensitive Drum Pads (on one chip Atmega328) ‘Very simple’

So I’ve been looking into how to do this, and I found someone on reddit asking this same question like 3 years ago, and they’re still active. I was planning to log in just to link them the video since literally everyone just told them to use regular buttons, but they obviously want to make the real thing, and it’s a night and day difference between using velocity sensitive pads and simple buttons.

Anyway, I realised giving them that link would be contributing to making reddit the go-to place for information, but I didn’t find this there, I don’t spend time there, and in fact my alts keep getting banned, and I’m the one adding the information.

So since reddit doesn’t want me, I figure the best way to solve this is to make a post here and link them to it. That way I’m helping them with their problem, adding content to the fediverse, and linking people here.

The only thing to add is that I plan to expand on this to make a proper MIDI controller using some of his suggestions for improvements, and I’ll be making a modular set of boxes that can magnetise together to arrange however we want.

Anyway, if that person or anyone else finds their way here, hello! Welcome, this is a much better place than reddit.