EDIT: I think this video shows a better design, although I note some improvements below:
Making a DIY analog force sensor under quarantine, with the Kontrol Freak. | KontinuumLAB
The main video linked uses two strips of copper bridged by the velostat, but this creates deadzones where those copper strips are, and probably also gives different responses depending on the shape of the region being pressed. I've done more research and a much more consistent method should be to sandwich the velostat between the two conductors so that the entire surface gives a consistent response that goes directly through the material. This should also give a more pronounced response because the length of the circuit through the velostat is only the thickness of the sheet, not the width of the pad. This should also make it less sensitive to changes in the pad size.
Some videos use conductive fabric, but the best one I found uses adhesive copper tape. If you're getting this, make sure to use copper tape that is conductive on the adhesive side, as not all of them are.
And a follow up video with a more refined method of building the pads and ideas about how to improve the analog-to-digital conversion:
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'
I suppose combining a piezo sensor with a simple touch-sensitive control might achieve a good effect, but velostat seems like a simpler solution to me. Also if you want a capacitive sensor on the surface you probably can't use the soft rubbery material that nice MIDI pads use.
Also this guy is quite good at his explanations and breaks down quickly how to make a full button pad, although he still uses regular buttons and pressure-sensitive ones would need a bit more logic to understand:
Launchpad || DIY or Buy || Keyboard Matrix & MIDI Tutorial
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. Also they said they live in India where a lot of musicians can't afford the more intuitive interfaces because they're massively marked up, and I thought they should have the information they need to make a DIY solution.
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 the second video's suggestions for improvements, and I'll be making a modular set of boxes that can magnetise together to arrange however we want. Also I'm going to look for translucent silicone rubber that I can illuminate with RGB LEDs so the sequencing can be animated.
Anyway, if that person or anyone else finds their way here, hello! Welcome, this is a much better place than reddit.