Considering the tensions the leech of an unstayed rig will see anyway I think we can simply attach the turning block for the spi halyard directly the sleeve itself. It will need to have a local reinforcement there and probably a strap inside the sleeve.

An alternative is to just have an opening in the front part of the sleeve and have an eye strap protrude through it on which the turning block is mounted.

A third alternative is to go for a mast top spinnaker and that is not a problem for a mast that is unstayed already. If we do that then this F12 will be monster downwind boat because that will increase the sail area by 2.5 sq. mtr. to a total of 11 sq. mtr. You'll be beating F18's on the downwind legs !

Personally, I like the block-stitched-sleeve and internal trap the best. Simple, cheap and doesn't affect the capabilities of the unstayed rig in any when the spin is not fitted (as in complete 360 weather vaning)

Wouter