Fill a piping bag, fixed with a small/medium tip or with a small hole cut in the tip, with frosting. We used this organic frosting
and it worked well, though it will not get stiff like conventional frosting. It was thick enough to hold things together though. You could try a buttercream made with honey and keep it refrigerated in order to avoid sugar, but it will be hard to get a stiff enough frosting without starch or sugar.