A random unit cell is shown in the next figure.
Random structure, 10 % porosity, (correlation level 0.0), cluster ratio 20
If you are working with cuboidal unit cells you are restricted to random structures.
In general, we would advise that you do not use this structure type, to the extent that it is often excluded for users without developer licences. Owing to the nature of the simplex, it has to be allowed to investigate non-random structures while working, then when it generates an output, it generates a random unit cell. So there is a discontinuity in the automated workflow which we have not yet addressed. Random structures are part of all other structure types as the Correlation Level parameter tends to zero, so there is no need to invoke this specific structure type. Remember that the Random structure type forces extreme randomness at the smallest length scale, which almost never occurs in natural samples.