bolster and strop

[osteoderm]

Pinch the grommet in the middle to form an assymetrical figure-8 shape (larger end around mast, smaller end for block shackle/lashing) using a round siezing. Serve the grommet well before siezing.
Brion or someone else here can probably provide some gee-whiz formula for your starting grommet size based on the diameter of the mast and projection of bolster.
Of course, the major drawback of this is that you have to have the mast freed up (ie., no shrouds, stays, fittings etc.) above the bolster, and then slide the grommet down into place from above. A well-made crane such as this should last a long time, so it's unlikely that you'll need to replace one while the rig is up. Of course, you could make the grommet up in place...