This bullseye has a single ring of projecting double tapered voussoirs. They have a 102mm face, and were supplied loose for building in around a circular window.
The opening was later infilled with zigzag brickwork, cut from brick slips mounted on a backing block, to create a gable feature. This was supplied with open joints for pointing up on site, and the brickwork was done by ourselves.
Bullseyes with a wider and/or bonded face are another option.