I would probably wait to see if anybody more experienced than me has input.
BUT
I'm pretty sure that the raised square detail is supposed to sit flush with the edge. You may need to trim that all down a bit.
As for the gaps that'll leave, you have a few options.
The first is the easiest, really, and I highly recommend it. Since you're going sandy, your backpack will cover any gaps. I have about a 3" gap between my back and kidney plates, but my pack covers it up!
Now, if you aren't going to have a pack, there are a few things I had thought of while making my armor. Note that last I checked, MEPD requires you to have a pack, even if the TD you're emulating doesn't have one. This isn't a problem for 501st acceptance, but if you plan on getting MEPD too, you'll need a pack!
Now, one option is to add shims. Same concept as what we *ahem* larger troopers have to do, just vertical. I did this to extend my ab and kidney plates using some ABS and Apoxie Sculpt to fill in the gaps and extend details. (yes, even after that I still had a 3" gap. I knew my pack would cover it though!)
A super lazy, and NOT recommended way is to kinda take advantage of the CRL. It states that there should be minimal to no GAP between the butt and kidney, however, it states that there should be minimal to no OVERLAP between the back and kidney. So teeeechnically, you can have as large of a gap between your back and kidney as you want. Again, I definitely do not suggest doing this and Jason could possibly not even allow it. He gets to make those kinda judgment calls! I only bring this up as a possible, desperate last resort. Besides, if your back is exposed it just doesn't look as good!