I've got a little amount of aluminum boxes: 8 More units are in the way...
These are made for the 2.8'' screens right?
Yes. This case fits with the PiTFT 2.8" and HifiBerry DAC+ (also pro and lite versions).
I'd really love one for the 3.5'' adafruit tft+ model! But I suppose these are quite hard / expensive to make?
Yes. It's not affordable to manufacture single units of aluminum cases. A minimum quantity is required.