I found that my plastic opening tool wasn't strong enough (it broke) to pry open this casing.

I used a 1.3 mm Flathead screwdriver as an opening tool.

After you have gotten under the edge of the back plate, slide your tool along the length of the plate.

Be careful if using a screwdriver to open the casing. While there are a minimal amount of components that can be damaged, caution should be exercised. It is not recommended.