Mounted these directly to the bulkhead and had no problems. Note that both halves of the housing go on the same side of the bulkhead, don't sandwich the bulkhead with the housing.
“Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body. But rather, to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming .... WOW what a ride.”