Fireplace corner insert?

I an building my own fireplace around cast iron insert with gl front and one side. The top of this insert has only about 10 mm thick rim around it, and I am supposed ti build a ledge on it. If it was not a corner one, it would have been easy - just to place a supporting bracket above it and lay a row of bricks on it. But since it is a corner insert, I wonder how am I going to build a 90 degree ledge on top of it. Any suggestions?