The layers can be held together with a couple small pieces of foam tape, dabs of hot-melt adhesive, epoxy, etc. If you added a replay button you can encase the whole thing in a plastic bubble (such as toy vending machines dispense) with just the button protruding.
The mounting holes on the LED backpack can be used for attaching a lanyard. Plastic lace from a craft store is one option. Do not use a metal chain for this…it’s conductive and could cause an electrical short as the pendant shifts around. Another option is a pin back…we have a magnetic type in the shop, or you can find the regular variety at most sewing or craft shops…again, be careful with this bridging electrical contacts.