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 a Oak treasure chest flatpack from scratch gives a profit of: -850
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.