Originally Posted by animenerd93
For the key, u prob won't b able to stick it in ur back.
Why not? Where there's a will there a way!
You can purchase pretty strong magnets for holding together costume pieces that need to defy grafity. It's a trick from my costume design professor. As long as your prop isn't too heavy and you have a proper mount for it attached securely to the costume, you can wear the key the way you like. Dizzymonochrome's strap suggestion is an excellent one and probably easier if you don't mind the straps showing.
Another possibility: for the little designs you can inject hot glue into fondant molds, which are inexpensive and easily available at Joann's, Michael's, or Hobby Lobby. It'll take a lot less time than sculpting each design individually. Then you glue them to the prop and spray paint it. (Here's an example: link
If you have patience for sculpting, my champion for smaller props and details is paper clay because it's smooth, lightweight, and inexpensive. Use a cheap watering can as a base so you start out with something sturdy and don't have to spend as much on materials.