Excellent! I'm glad that it wouldn't go to waste. Have you ever found a good looking, cheap-ish body donor for a small planar driver? But then I think the old planar drivers aren't that efficient. I can remember back at Malaysia, that I can drive my YH-1 pretty okay-ish using my phone (almost at max volume but not at my normal loudness).
not so much on the cheapish side since I had to buy them new. But the interior is spacious enough, and if you're using a <40mm driver you can basically just pop it in after removing the dynamic drivers (hard part). The spaciousness is attributed to the cheap plastic cups with no stiffening reinforcements on the interior. Naturally 0 damping inside either. For slightly larger drivers, say the HP/YH-2/3/50 as long as it fits in the baffle there should be ways to make it work since the holes in the magnet span within 35mm.
The other often used one are those Sony ZX-100. With custom baffle I think (at least that's what ludoo uses).
I think vintage headphones that are bought at a bargain come to some of the best cheap donor. That's how I got the one keeping my HP-2 drivers. Those are open back though.
It didn't even take 5 minutes and I fixed the bigger sonic issue with them with nothing more than cotton!
+1 @skip Apple rant.