I'd try to get your foundation (plywood or pallets or whatever) up off the ground on cinder blocks. That space underneath REALLY helps the lower part of the pile get & stay dry - good for the foundation too. The more space the better.
I double stack on elevated pallets - no tip overs yet after all these years (knock on wood...). Double stacks are usually somewhat more stable.
I'm sorry. I was unclear. There is plywood on the ground, cinder blocks on top of that, and landscape timbers on the cinderblocks.
My eighty year old mother came to visit today, and she has cut, split, and stacked all the family's wood for the past 40 years (thankfully now she's getting a little help with the big stuff from a men's group at her church since she has a bad arm). She looked over the rack and is going to consult if we split enough wood tomorrow to commence again.
I did feel that it was a victory that she's now talking about building a holzhausen with some wood she needs to stack and doesn't have room for in her primary seasoning or storage spots. She examined mine pretty thoroughly (corrected one piece she felt was not secure enough) and even stepped on the side to look inside.