I think I finally figured this out, after trying all kinds of wraps, paper etc, plus double bags.
Last year, I froze my PWS spot shrimp in ziplocs (poor man's shrinkwrap style, submerge bag below the last little bit of zip top, then close that). Then, after they were frozen I felt along each bag for the spines poking thru and bopped them off with a small hammer---a jar or table knife butt, anything hard would work. Then I opened the ziploc leaving them in place, and vacuum packed them, and so far I haven't found a bag with a broken seal = problem solved! I guess you could also vacuum pack them twice but the ziplocs are less expensive, and it works great.