I haven't made one before, but I think I would go about it this way. Bake a round cake of choice. Carve the sides so they angle down. Crumb coat. Make snakes of fondant that gradually taper at one end. Place the snakes from the top edge of the cake down to the cake board (tapered end at the top, larger end at the bottom). You may need to manipulate the fondant so it lays how you need it to. This will form the ridges around the bottle cap. Cover in fondant.
Definitely post pictures! This is a great idea, my hubby would love it (he is a beer lover and collects bottle caps). I will have to do this for his bday in July.