Actually, just use the zip ties mention as above throughout the corset (you'll see that after an hour of wear, the ridgeline does crap). You can probably take and inch or two off the bottom, and perhaps two to three off the top.....it's too big there. If you want to, you can also take a straight .5 to 1 inch off EACH side of the center back....that'll give you roughly a two inch gap in the back, and with the previous adjustments, everything should be non-floopy and stuff.
Very nice for a first mockup. Corsets aren't easy, as fitting them correctly is the hardest part.