Simply put, unscrew the three rear screws.
Gently pull the back rear of the case up (you may need to flex it to the sides a little) - it is tight but will come off. Be VERY careful of the exposed power supply board.
Once off, the Sony drive unit is on the left held in with four screws.
If you unscrew all four screws on the unit you can lift it up CAREFULLY as there are three ribbon cables. You can then put some padding under the unit, and where the screws go into the case (between the optical unit and the case) and tighten up.
You can also, if you want, put some padding on the inside of the top case which will touch the optical unit when it's back on. Bit of double sided tape helps there.
I used something similar to this :
Tenura anti slip fabric roll - Non slip mats
Can be trimmed and isn't thick.
Replace screws and slide case back on (it's tight). Try it out...
You can test it with the case off but don't pick up the unit with a disc in in case it jams.