Pedro Morales was in Dallas recently and shared this method, so I will share it here. Some things of note, Pedro is from Puerto Rico and thus works almost exclusively with topicals and very few plants not native to the island. After years of fighting to have Pines and other non native bonsai subjects he too has embraced the path of least resistance and grown to love the native material and thus eliminate(to a great degree) the bonsai shuffle and the fight we all all have with species that do not thrive in our local climate.
Pedro said he grows his own yamadori in the backyard now, the searching is much easier this way. He has 40 some odd plants growing by this method and finds that in 2-3 years he gets trunks that are the size of 5-6 year plants he was growing in ground before. Old tires have been the answer. He places landscape fabric on the ground then stacks two/2 tires on top of each other. He fills with a 50/50 mix of sand and organic material, no sifting, no fuss, nothing special. It can be growing mix, landscape mix, manure, compost, he does not care and regular sand. He is able to manipulate them and branch select and prune while growing and when time to “collect” he takes a saw and saws between the 2 tires.
He believes that the black tires is the key. They warm up in the day and provides additional warmth at night. Roots like warmth and the tires make a sort of heated root bed. The sand holds additional water and also provides a buffer for excess heat, but allows for easy root growth.