I cut the bricks that I had on hand today. She's not pretty from the side due to the brick layout but the riser is nice which is what matters! I decided to put in the extra effort and cut bricks to make the riser lock into the fire box also.
Port ended up being 59mm rather than 50mm but not a problem I think.
I need to collect another 24 bricks for the fire box walls and the floor to finish it.
From here a plate will go on top with a hole over the riser to insert the insulated flue pipe riser.
Will probably enclose the entire core in a steel box and slide it through into the bell and seal the edges.