cu. ft. X 1728 = cu. in 1728 is the number by taking 12 to the third power. In english terms it is V = H * D * L (1728 = 12 * 12 * 12).
Then take that number to the cube root or cu. in.(1/3). This will give the multplier for a 6 sided box. Then plug in either the golden ratio (0.618:1:1.618) or acoustical ratio (0.7937:1:1.2599). You can make a cube like box or a custom ratio because box modes will be higher than the used frequency spectrum.
It is mostly geometry skills. Then add 1.5 to each final measurement if you are using 3/4 MDF board because two 3/4 in thick board is 1.5 in.