You will need 4.5 yards to cover both sides, to be safe. 108 trimmed down to 78" leaves 30" to be used on something else.
78" for both sides, allowing for generous seams/hems. 78 + 78 =156 inches divided by yards (36") = 4.33 yards.
you already have the 78" across with the 108"width. So what you need for each side is 78" . That is the 78 x 2 = 156/36 = 4.33
Good luck. Sounds like a great project.