...just in case you're in need!
Have you ever heard of self cling window film? It comes in lots of "stained glass" design panels, and you stick it on your window by just wetting it with soapy water and smoothing it on. I got it from here and have just fitted some on my daughter's window:
So where's the challenge?? Well, I have some film left over, and can't bear to throw it away. I have just been experimenting and have found it die-cuts beautifully and you can punch out shapes as long as the punch is quite strong and it is still on its backing paper. You could definitely cut it into shapes and apply to windows (maybe spelling out a name?). I think it would stick to acetate really well. I was wondering what else my readers might be able to think of to do with it? Put a suggestion in the comments, and I'll send you a piece to try it on!