Wide, hardwood shims every 5 to 6 inches and you're good to go.

Except once in place, I won't be able to get to the back edge...but I think I should be ok, the reaer centre is sitting solidly, so shimming the corners should suffice. I hope :)

The 4 corners are more important to be solid than the center, I really believe that doing the corners rigidly is better than shims that may give and leave the center as the main support.

thats what you said last time..

Steve

This time I built a false back, so it's right against the wall. Once in place, there's no getting behind it.