I have seen the same cake, I am making the shells out of gum paste instead of the chocolate they will travel better and I have no worries of them melting. Gum paste is the stuff the flowers and roses (dried) are made of on cakes. Available at michaels, just add water and mold to the shape you want. I bought chocolate molds and I will be using them.
I tried the chocolate thing last weekend on a birthday cake and we placed the cake in the sun, the chocolates melted, I had put was in the chocolate and either I did not put enough in or it just wasn't meant to be.
Dea