Verdatum pretty much hit it on the head. Reinforcing the inside is the only way to retain the current detail. If you add coating to the outside (resin, mod podge, gesso, etc), it's only going to soften the details.
One method that I can think of, given the material of the prop, is on the backside, carve a channel all along the ring. If it's cardboard it shouldn't be too difficult. Then, take some thick gauge wire and bend it into a circle. Glue it inside to provide more support, and then reseal the channel. That should at least give it some more support and keep it from breaking apart completely.
I'd also reinforce the ring on top that the cord is tied to. That is going to be under the most stress as you wear it, and if that ring fails and breaks, the whole prop could fall and be ruined. Above all else, make sure that ring on top is strong and secure.
The only other suggestion is to just treat it like your baby and handle with extreme care.