A mahogany fancy dress box flatpack can be made at a workbench (Bench with vice or better) in the workshop of a player-owned house. Like when making the normal Mahogany fancy dress box, this requires 80 Construction, uses 2 mahogany planks, and gives 280 experience. The flatpack can then be used on a fancy dress box hotspot in a Costume room, either in building mode or not. This requires no Construction level and gives no experience.
The cost of making and selling a mahogany fancy dress box flatpack from scratch gives a profit of: -2,309
After completing the hard Fremennik achievements, they can also be traded to Advisor Ghrim in exchange for 338 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.
See Fancy dress box for a list of armours that can be stored.