My method is this. Fill fondant into standard size dishes (I use the deeper Chinese take-away type). I cut a hole in the insulation sheet (placed over the crownboard) for placing the inverted dish and then cover with another sheet of insulation over that.
Keeps heat in, even when the plastic dish is nearly MT, one can see the rate of usage by simply lifting the roof and top insulation sheet, and is quite easy to replace with another dish as necessary. The top sheet would also prevent the fondant getting too hot if the sun warms the roof space.
Disadvantages? - possibly the relatively small amount fed each time, but with not so many hives requiring this approach, and the need to keep an eye on the hives, if they are short of stores, means it is of no great importance.
I covered the dishes with cling film and then fitted lids for storage, so the fondant did not dry out before use;I think I removed the lid and cut a slot in the cling film for use but may have fitted spare lids, with slots cut in, for the bees to gain access. The cling film prevents a sticky mess if it is not used by the bees.
The cut-out pieces of polystyrene are marked for easy replacement into the same holes, in the same orientation, from whence they came. Marked carefully because they are easily mixed up and if then forced together some will never fit together properly.
Regards, RAB