A great idea to transform an old stuff: CUECLYP's phone case
"CUECLYP's iPhone case is made of TPU and PC materials, and has strength and elasticity. We attached a part of discarded PET poster after exhibition, performance and advertising on the back side through collecting, cleaning and cutting manually. You can remove the D-ring patch on the charging terminal. Or you can use your phone more comfortably with the D-ring and hand/neck strap. all double-sided tapes we used are 3M tapes.
(Straps are sold separately.)"
They provide lots of upcycling products other than phone cases, please visit their site and have a look! They ship worldwide!