Have you tried the clay granules sold by Klassman ?
I'm not sure what that is. Do the granules support capillary watering or do they disintegrate to act as a wetting agent?
I've looked it up and I'm still not sure.
Since I have clay soil the idea of buying clay goes against the grain!
I know, I know, I'm being inconsistent since I used to buy John Innes, but I convinced myself I was getting a sterile, fertilised and pH balanced product that wouldn't downgrade the soil-less compost I was adulterating.
I would be tempted to use burnt and pulverised subsoil if I was doing it now - the only good use for a bonfire in my opinion (apart from making charcoal/'terra preta' - but that's another story).
Cheers.