After defeating the Mankey Gang in Uproar Forest, Jumpluff will give you one Peeled Chestnut. Give it to the Mankey Gang to pay for construction of your team base. You only need two more Chestnuts from Uproar Forest. You usually cannot find them until Floors 7 or 8. Your Pokemon peels a Chestnut to make it a Peeled Chestnut. Once the construction is complete, your base will look like your Pokemon's head.