I like citrus and poultry, so I often put fresh grated zest in brines for chicken.
I have a spreadsheet I use to calculate a 3% salt solution for slow brines. For two 4 pound chickens, and a gallon of water, you would need 6.5 OZ (184 grams) of salt to make it 3%. I don't like to put sugar in chicken brine so that I can crank the heat.
I often do something like this:
1 gallon water
8 pounds chicken
*6.5 OZ salt
zest from 1 orange or lemon
bay leaf
10 peppercorns (black or combination)
5 whole cloves
1 t. fresh rosemary, minced
3 cloves garlic, smashed
1 T toasted and fresh ground chili (Ancho or blend of several different types of chili)
*Note, if you change the amount of water or the amount of chicken, then the amount of salt would have to be adjusted to maintain the 3% solution.
Toast chili for about 60 seconds in dry pan over medium heat, grind, then set aside. Toast peppercorns and cloves in the same way and lightly grind to large granular size in mortar and pestle or spice grinder.
Bring two cups of the water to the boil and add salt. Stir until it is well dissolved. While the water is still hot, add chili, peppercorns and cloves. Smash the garlic with the side of a cleaver to release the flavor and add. Zest the citrus directly above the container of water so that any oil is captured. Add bayleaf and minced rosemary. Allow this to steep for 15 minutes, then chill the mixture along with the remaining water. Alternatively, instead of chilling, combine the two cups brine/spice mixture with 64 OZ ice and 48 OZ water.
Then place the chicken in a large food-safe container and add the chilled brine mixture. (Never add chicken to warm brine.) Keep in the frig for 4 hours, then remove chicken, rinse with water, and pat dry with paper towels. For best skin results, air dry uncovered in frig for 4-8 hours or place in front of fan on high speed on the table (with a drip pan underneath) for 30 minutes while you are getting the grill or WSM ready.