If it comes off that easily with just a little heat, no wonder the pad doesn't hold during use since those FETs can get pretty hot. Don't know if you've seen this, but the pic below shows the proper dimensions for the new spreader plate when not using the thick thermal pads:
Yep... It was quite disturbing to see how easy it it came off when I used the heat gun... I saw your diagram before when I started researching on this mod... nice illustration. I have a small sheet of 3/16 aluminum
(not much thicker than 1/8) That I think will work just fine after some light sanding and polishing. I am not sure if we have the popular 'ARCTIC ALUMINA' in the island... so I will have to settle with the generic brands.