I did mine today,I did not felt it was safe to leave the underside unprotected so I just left the upper part of the GPU uncovered(my problem is on it,and it's a falcon,so I didn't feel any need to reflow the solder on the cpu).
I used much more t-shirts then you did,I've used six.Instead of using thermal tape I would reccomend using a micropore surgical tape,it didn't smoke here.As I left only the upper part of the gpu uncovered I left it on a pre-heated oven for 17 minutes at 255°C,I think unlike you guys I used a kitchen stove oven,so it's much bigger than the motherboard.
I can't say it was sucessful yet,as it was working before,I did the x-clamp mod but at this point I would have to heat the board by disabing the fans almost every other day.At least it's still working.