Read the back of the box and see how far away it says for max distance. 12" may be a tad far for that bulb. Having it on top of the screen is a no no. That screen may not block the light but it's a proven fact that it blocks a significant amount of the UVB rays, which is the entire reason for that bulb. A 5.0 bulb barely has any UVB to begin with and if you mount it too high and on top of a screen you might as well not even have one. Check the manufactures heights but you'd be better off finding a way to mount it under the screen.
Those are good idea! On the box it says to leave it on top of the screen and not inside the enclosure, because it can overheat the tortoise, which is why I did what I did. I've attached the two diagrams that were on the box.
I have a closed changer and mounted mine to wood but I'm sure you could use zip ties or metal tie wire. Like the thin wire you can buy to hang paintings with.
Mine was a plexiglass cover. Same idea. I had to take it apart a little and drilled small holes in the plexi and the light fixture, then ran some screws with washers in. I took it apart to make sure that the screws didn't short anything out. You may need to make a support for on top of the screen to even out the load and weight of the light so that the screw holes don't rip through.
Zip ties sound like a good option, but screws will last longer and hold better. IMO.