We retract our lazy jacks before putting on the cover and then hold them off the mast with sail ties when needed (most nights at anchor, less often in the marina). On ours the forward side of the jack control line doesn’t slap, just the part aft of the top block. Ours are the PTR style shown in the youtube video above, and work well. Purchased before leaving Washington for Mexico at a point when time was more valuable than money, but I don’t think they’d be hard to make up just time
consuming.