I suggest using Ergun equation!

Please refer to Applied Mathematical Modelling 37 (2013) 9052–9062

Is Ergun equation applicable to random packing?

It is according to the void volume of the column, so, doesn't matter to be regular or irregular!

If the column is countercurrent, then you might try using any one of the available forms of the generalized pressure drop correlation (GPDC). This correlation requires only a single parameter, called the packing factor, which can be readily found in tabular form for most common random packing styles and sizes. You can find information about the GPDC in any number of textbooks, like Kister's Distillation Design ISBN 0-07-034909-6 or Ludwig's Applied Process Design for Chemical Engineers, Vol. 2 ISBN 0-88415-025-9.