I have one of these on my GS3 and plan on getting one for my G3. It's cheap, and not bulky at all.
I'm really liking the Diztronic case. Case doesn't add much bulk and is actually pretty good looking.
Also took some pictures. Apologies if some of them come out a bit dark.
I don't know if I can post the link correctly, but I went with the Diztronic case from Amazon. I'm hoping that it will keep me satisfied until the LG Slim Cases become available so I can use wireless charging!
Although you're looking for a new back plate, here's another option. Consider getting a case. I just got this and it's great. It had a great feel to it, and it is not thick at all.
I started with this case but after less than a year the back got very scratched up and discolored. Switched to the Diztronic case and I am liking it a lot so far. Good protection for my needs and less bulky than the Spigen.
This is the one I've had on my G3 for over a year.