Use O-rings, flat seals are not so good as they will not fit in the groove and may leak. O-rings compress easily and fill little imperfections that flat washers will not fill without the use of a sealant and some sealants will make it hard to remove the wing at a later date if they leak again. I fixed mine six plus years back and I have had no leaks.
The seal around the mounting plate is crucial and is originally a poor design. The new seal should be fitted so that no pockets of water sit under the plate. Some people have used silicone sealant but if it leaks because they did not put enough sealant in the varying gaps it will be hard to remove the plate to fix the problem.