Just saw your updated post. I still wouldn't put it on the inside of your glass. I would recommend putting it on the outside. I say this for a couple of reasons: 1. for cleaning practicality, it will be very difficult to keep your flag clean and keep the glass clean too 2. it may leach chemicals that could make your fish sick. If you don't want to attach it to the outside back of you tank, then I would say painting the exterior may be an option that you should look into. I haven't done it myself, but several members here have painted the back exterior of their tank.
I currently do have the outside painted with acrylic paint and thought about taking off a section of it, laying the decal and repainting over it. Probably what I'll do I just really don't want distortion. Thank you for your input Red