I'm not worried about a soldering iron damaging the FLIR, although something really hot might. Ii just saturates somewhere above 200C.
The black trash bag thing might work to extend the range a bit. A few mils of bag is almost transparent, so the layer count might be tuned.
I'm guessing that a camera-type IR filter won't help, but I'll try it. Somebody else has the FLIR just now. It's pretty popular.