A guy on YouTube, Ant Invasion, made a tutorial where he used sand rather than modelling clay or similar to mould his chambers, his reason was because he could wash the sand out rather than having to remove the hydrostone/plaster to take out the clay, of course his chambers were perfectly coated with sand. Perhaps you could adapt the method to suit what you are doing, maybe coat your mould with sand then when the hydrostone is poured the sand will be imbedded into the hydrostone when dry?
Just thinking out loud really.
I might try this sometime as a small experiment, but my main concern is that it will make it near impossible to get the clay out afterwards, or that the clay might pull the sand back out.
Edited by AntsCalifornia, December 7 2018 - 4:50 PM.