A greenman's ale (barrel) flatpack can be made at a workbench (Oak workbench or better) in the workshop of a player-owned house. This requires 26 Construction and 29 Cooking, uses 3 oak planks, 8 Greenman's ales and gives 184 experience. The flatpack can then be used on the barrel space hotspot in the Kitchen while in building mode. This requires no Construction level and gives no experience.
The cost of making and selling a greenman's ale (barrel) flatpack from scratch gives a profit of -8,573 coins.
After completing the hard Fremennik achievements, they can also be traded to Advisor Ghrim in exchange for 86 coins being added to your kingdom's coffers in Miscellania. See Flatpack: Miscellania reselling for details.