If you are going with a magnetic surface + glass, I don't how a thermal pad would work in that mix ( I missed that in reading your post - my bad ...).
In my case, I am just using glass directly over the heater plate. I was talking about this stuff:
https://www.amazon.com/gp/product/B01N9HGKR1It comes as a 400x200mm sheet, you have to cut & piece to get full coverage. It is very delicate stuff, you do your cuts before removing backing material & only remove backing on one side prior to placing (or remove as you are laying it down). You remove the other side once it is in place.
The glass rests on top & suction keeps it in place. To remove the glass you break suction at one corner (a twist with your part removal blade) & slowly lift away as it comes free. Again, I don't know if this would play well with the magnetic surface.