Window Jamb Extension

I need help with a basement window jamb extension. The wall in front of the window appears to not be square with the window and depths range from 14 3/4 to 15 1/4. Since the depth is greater than 12", I purchased laminated pine planks that were 18" wide.

Here is my plan: Since the box needs to be square, I was planning to build it based on the largest depth at 15 1/4 and add for a 1/4 reveal all around. I was going to install and shim temporarily and then use a pencil to mark a flush line all the way around. Then, I plan to make the unit flush with the drywall by using a hand planer.

Now, is this what you would do or is there another way that I am not aware of? Should I just plane the jamb when it is attached or remove and work on it? Any help would be appreciated.

-Brent