I have to stack 6 of these then sit it  on it's side.  They are 14" inches round.  What is the best thing to hold the layers together?  Royal icing or buttercream? Thanks!!!

RI would work - better than buttercream.  Personally, I'd use melted chocolate to "glue" them.

This appears to be a disk of Rice Krispy Treats???  If so, I agree with @640Cake ‍, I would use melted chocolate, especially for turning on its side and/or carving. 

Thanks!  I thought chocolate would work best but we are still a week and a half out from the party and I was concerned that chocolate would mold if not refrigerated.

Yes it's made in a 16" cake pan that was a typo in my earlier post.  I am turning it into a bass drum and it won't be eaten

I have modeling chocolate figures that I've kept for three years and they didn't mold.  Candy bars in the store do not mold, nor do my chips and baking chocolate.  I don't think you need to worry about it molding.

Great thanks for the help!!