I have a difficult retrofit to bid. Multi-tier retaining walls with railroad tie steps in the middle. Walls are landscape timbers and they want to maintain that look, much against my advice. Its their place though. I did convince them to go away from the ties for steps and would like to put in stone of some sort. Problem is, there are 28 steps at 6" and the treads are winding. Depth of tread is between 8" and 20". Don't know of any 20" deep products. Any suggestions?