This is the type of skirt I had on mine and yes it was attached to the fiberglass bumper....Don't Do It This Way! When the back end came close to the ground it would bend and rip out some of the rivets holding it on cracking the fiberglass at the same time.
The best would be as stated by fixing it on an angle iron and fixing that to the frame. Try to get it close to the ground without touching.
I don't believe what someone said about them being useless, but they don't work as well as the rubber style and they're not the same price either.
I'm sure what you have chosen will work well.