An oak treasure chest flatpack can be made at a workbench (Steel framed bench or better) in the workshop of a player-owned house. Like when making the normal Oak treasure chest, this requires 48 Construction, uses 2 oak planks, and gives 120 experience. The flatpack can then be used on a treasure chest 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 an oak treasure chest flatpack from scratch gives a profit of: -896
After completing the hard Fremennik Tasks, they can also be traded to Advisor Ghrim in exchange for 56 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.
See Treasure chest (Construction) for a list of armours that can be stored.