How to make a polyester blanket thicker?

Hi, I ordered a blanket for my fiance for christmas, and it is 100% polyester and extremely thin. I was expecting it to be a little thicker, so are there any ways I can make it thicker? I don't want to do the tie blanket idea because my fiance is 6'4'' and the blanket is only 40x60 so making it smaller isn't an option (granted, its only a "couch blanket"). Is there a way to sew a piece of thick fleece to the back of it securely? Serious answers only please, and tell me if there are risks of the fabrics tearing with your ideas! Thank you so much!