Ok, so long story short, I have never even picked up a saw before a couple months ago and I decided (like an idiot) built in bookcases would be a great starter project. Some plywood, some screws, it'll be done in a few weeks. I hang my head in disbelief now, but I am learning and enjoying this so much!

The visual: I have a 12'x12"x12" platform with electrical outlets built and wired and almost done. Pine 1 by as a front kick plate which will be covered by 1/4" oak veneered plywood later this week hopefully. On top of this platform I have 4 boxes that are the bookcases without shelves. They are 7' high and go to the ceiling. The boxes are 2 on the left, 2 on the right with a 2 - 2 1/2 foot gap in the middle for a window and a window seat in the future.

The problem: The gap in the middle. I am putting 1 1/2" trim all the way around just the book case, to cover the exposed plywood. There is exposed plywood inside the gap area between the sets of shelves, so I want to return the top trim back to the wall to cover it. BUT because there is a seat in the middle, I do not want to just extend the top trim 1/4" over the side to cover it. So with my extremely limited knowledge on this subject, crown moulding seems to be the answer to allow me to return the top around the corner, but keep it flush with the front side of the box.

The question: Can I make crown moulding out of 1 1/2" trim, or do I need to actually buy crown moulding that is already cut at an angle and all? Also is there some other simpler answer I am not seeing to my dilemma?

Thank you all who read the small wall of text, I really do appreciate any help!!