A dragon bitter (barrel) flatpack can be made at a workbench (Oak workbench or better) in the workshop of a player-owned house. This requires 36 Construction and 39 Cooking, uses 3 oak planks, 2 steel bars, 8 Dragon bitters and gives 224 experience. The flatpack can then be used on the beer barrel hotspot in the Kitchen while in building mode. This requires no Construction level and gives no experience.
The cost of making and selling a dragon bitter flatpack from scratch gives a profit of -2,422 coins.
After completing the hard Fremennik Tasks, they can also be traded to Advisor Ghrim in exchange for 109 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.