I make the mashers just like I would for say Thanksgiving. After mashing I put them in the fridge to cool for 30 minutes. After 30 minutes I mix in about 3/4 cup shredded cheddar and three sticks of string cheese cut into 1/2 inch pieces. Then back in the fridge for a day. Next day soak a cedar plank for an hour, form potatoes on plank (stay away from edges as they do expand) and place on grill over med. heat 30 - 40 minutes until outside has formed a crust and inside is hot.
When not planning ahead enough I have put them in the freezer for a couple of hours to rapid cool.