I have had leak problems in the past even with a stock spoiler..
Solved it very easily. When you put the new spoiler on, take some black rtv.. Place the spoiler on trunk lid with bolts through holes. Fill area of hole around bolt/stud with black rtv. Put nut on and tighten.. It will dry. Be soft enought to not interfere with taking it off.. Will never leak.. Hell, I even did somethign similar with faucet washers/gaskets as some people did with garden whole washers/gaskets to keep the spoiler a few mm off the trunk lid (no more paint scratching).. .. The two combined, no paint rubbing/scrathing and absolutely no leaks.
