There seem to be several options when it comes to finishing the firebox. Top loading fire-boxes seem popular in Australia, while the examples I found from the UK were mostly side-loading, capped with a half arch. There are good reasons for each and ultimately I chose what worked best for me (which seems to be favouring the easy road…).
I used four huge tiles (18″x12″) to span the top of the firebox. Since the tiles were 21/2″ thick, and I’ve been using 3″ series bricks, I had to fill the 1/2 ” gap with castable (see photo below). This solution proved to be a good one later on when I decided to create some primary air openings on the top of the firebox, but I’m getting ahead of myself.