An oak armour case 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 armour case, this requires 46 Construction, uses 3 oak planks, and gives 180 experience. The flatpack can then be used on a armour case 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 armour case flatpack from scratch gives a profit of: -1,409
After completing the hard Fremennik achievements, they can also be traded to Advisor Ghrim in exchange for 84 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.
See Armour case for a list of armours that can be stored.