I discovered a small rectangular piece of rubber - it squeezes in between the back of the plastic bracket and a tooth that sticks up from next to the back plate. It has weak adhesive on one side. It is a nightmare.
I found there were some details lacking in this teardown, and as soon as I removed the top half of the case, I realised my PS2 was quite different inside. I did my own teardown with pix. Want it?