I don't know if it's been awhile since you looked, but prices have dropped if you don't look at the name brand:
Bought one of these for work (HIKmicro, 160x120/19200pixel) for ~$300. It works, it's quite good compared to any thermal camera I played with 5+ years ago (even the fancy ones).
you could make your own thermal sensor, which is what i thought the article was going to be about from the title
this is a bit like 'i made a working, drivable car from scratch using only basic hand tools, raw materials, and a 2005 honda civic' or 'i built my own operating system around the linux kernel'
One could make an array of thermopiles, like the hacker that made their own imager out of discrete diodes (digiOBSCURA) . But each pixel would cost $7.
One might be able to make an array of thermistors (possibly with active cooling using a peltier) like the diycamera (digiOBSCURA) below. Might be an application of combining many RC oscillators in a tree and recovering the signal with an FFT. I have a gut feeling this is possible, but haven't show it. Isn't this the same as or similar to your keyboard multiplexer design?
Above is a seminar on analog layout designed to target Tiny Tapeout that went over Magic (by Jonathan Edwards) and Klayout (Thomas Perry). Sky130 and tinytapeout can do analog, but most of the tools and the examples are digital based.
Bought one of these for work (HIKmicro, 160x120/19200pixel) for ~$300. It works, it's quite good compared to any thermal camera I played with 5+ years ago (even the fancy ones).
https://www.amazon.com/HIKMICRO-Resolution-Portable-4%C2%B0F...
It's important to note that camera sensor tech has been advancing quite rapidly these past two decades.