Take a ranger band and glue something rigid that is the width of the ranger band, and the length of the items you are wearing (As they would be if you laid the items side by side.).
That part of the ranger band that is not glued to the rigid piece should provide enough stretch to hold the items together in a flat form.
If one ridgid piece is not stiff enough, you may have to glue one rigid piece on each side of the ranger band, but the fitting process might be more involved.
The other option is a short length of cartridge carrying webbing that you could insert each item into with the webbing attached by a small string to your necklace so that if you have all the items loose, you won't lose the webbing.
Bountyhunter