Hi Bastian;
I'd have to see the finished figure with the fire stream completed rather than the 'Photoshopped' version you have in the second photo. As it is, I really don't find the effect all that convincing actually.
In reality, the burning fluid stream coming from a flamethrower is actually much longer that you've portrayed. It's under pressure and shoots forward quite far and generally doesn't have fingers of flame licking out at the end.
I hope you realize that I'm just trying to be helpfully critical here and not knocking down your work. You're a brave modeller for attempting to render fire.....flames, smoke and fire are one of the most difficult effects to pull off convincingly and I applaud your efforts for trying. It's not ever been something that I've been able to do.
Having said all this, the technigue you're on to with the hot glue gun might very well have something to it. You've now got a medium that you could drip and sculpt while it's warm and you may be able to form it into a more convincing shape. Keep trying, I think you're onto something here.....
Bob