Well it took two days but we got the panel detached from the steel frame. First we had to use a sawzall and disc grinder to cut away the edges of the steel to allow a straight shot under the plastic panel. Then came the heat gun heating the steel to the point where it allowed an old butchers knife to slide in between the panel and the steel frame. On the middle areas where the plastic panel curves over the edge of the steel we used the windshield removal tool: a wire with two handles, to slide-cut the adhesive once the steel was hot enough to release it. The heat makes all the difference. Without it you could be there for a week with that windshield tool sawing away..
Here is the final result:
