Was there no need to secure one section of riser to the other? I suspect not, but no mention - so I ask.
I feel sure that two layers of 3/4" flooring will keep them in place, and as long as they're shimmed well, no one rocks or wiggles.
And I hope there is someone around to take a few pictures of the ceremonial sanding.
Edit - you know, among all the "show me" threads, there ought to be one to show yourself hauling sand.