Look up Mr.Canuck he does a coco mix with some amendments and Gaia 4-4-4 with some pretty crazy results. Pretty much water only. But this needs to be treated a bit differently. No runoff and if you get your mixture wrong you will be fighting it all grow long.
I'm trying this: 35% Perlite, 35% Vermiculite, and 30% Coir. I've added Myco (because I have some( and some Azomite. We'll see how it goes. Haven't done soil-less in a long time, but... ;) Also, had some water beads, so, they went in as well. Nontoxic, of course.
I just planted some and, I'll say the drainage is excellent. ;) It does hold moisture, but, we'll find out exactly how much later. Perlite and Verm hold moisture pretty well, they will wick it up from the bottom.