I need to glue MMF to the inside of a rice krispie made lid. The MMF will be facing down so I want to make sure it is suck well. What should use?

There are a few options...buttercream, royal icing, melted chocolate or a gum glue made by mixing tylose and water together until a thick glue forms.

Just dry it facing up until it's solid.

Thank you for your ideas Will buttercream dry strong enought to hold it?

It depends how much time you have to let it dry...I've used it on propped open box lids and it held, but if it's really humid where you are you may want to use royal or melted chocolate....or the glue.

Ganache also works really well

I've used candy melts. They also fill in the bumpiness on the RKT so the fondant will lie smooth.