My son came up with a simple and workable solution after using the boat one day. I have the deployable jacks well-described in other posts in this tread but without the (very good) method of mounting them to the spreaders. The working part of the jacks go up inside the mast then out through some bulls-eye fairleads. My solution to mast slap was to criss-cross the jacks around the mast. The mainsail cover went on in the normal way. It was awkward to do and pretty cluttered. What my son did was tie the loose parts down to the rigid vang about a foot back from the mast, put on the sail cover then tighten everything up. The jacks were held apart and away from the mast. Since then I added a hook to the vang and it works well.