I've used this technique on paper many times and love it but found that the shaving cream smell stays with the paper for a long, long time. I searched and searched for an unscented variety but the best I came up with is a raspberry scented cream that eventually dissipates.
I LOVE the box idea - have several of these in balsa wood and hadn't decided what to do with them. I'll definitely try out your project.
Have you thought of using embossing powder on your box? It might be fun to use clear ink and powder to give it some hidden interest. sab