Just sharing.
From my experience from using the stock aluminium heat sink, and following the recommended mod by Chris, I find it that the 20 x 20 x 1 mm copper shim is to thick, causing the stock heat sink to bend when screwed back to the device. Further more adding 1 mm thermal pad on top the heat sink and closing the tablet case I find the overall mod is way to thick, it can be seen very clear by naked eye it cause unevenness on the back case, so I immediately open the case again an remove all the mod. I’ll try the same mod again but by using a thinner copper shim (0.5 mm) and also a thinner thermal pad (0.5 mm) and see how it goes. Hopefully it fit.