I have to create a two-tier cake covered in white fondant with three colors of rock candy all over the sides of the tiers. The customer is picking up the cake instead of having it delivered. SO I want to be sure that however I attach the candy, it will stay put. 

At first I thought just plain ole buttercream but not sure. 

Then I thought royal icing but won't this dry hard on the sides of the cake? Doesn't make for a great bite of moist cake. 

Clear piping gel- but will that really hold the crystals in place as it travels in a car?

Has to be something that will work against the fondant and won't show. 

Cake is for this weekend. Need some good advice. 

THANKS!