Gaps i between wall and roof

I would cut block and set it in the opening. The smaller top gap would be covered with a fascia board.
Ron

Small sheets of tin, carried by your block supply company in boxes of 200, are designed to be placed between the next to the last row of blocks and the final row. They stop concrete from flowing down the cavity. In fact, they are called "Cavity Caps".

But if it is not possible to make these work because they are awfully lightweight, and can "float" up from the force of poured concrete if not locked between block rows (courses)..... (the block supplier will give you a handful to try for free), then just wad up stiff paper (we use concrete bags), and stuff them a few inches (tightly) down into each cavity. The concrete won't flow past.

Thanks

Indeed, a random mix of cement bags, small rocks and cement did the job perfectly!
Many thanks.