I've seen people fill in the holes on their front bumper (after taking off a front plate) use plastic/rubber plugs from an appliance store, like the kind they use on a fridge when swapping the doors to open the other way. Most are white, and can be painted to match your car.
Not sure how it would look on the decklid though. How many holes are there from the wing?
I had my wing off for about a year and just recently put it back on...For the holes, I went to the dealer and got some of the interior molding "X-mas tree" looking plugs that hold the pieces to the car...they gave me 4 of them for free, but I think you can get them at autozone also for a minimal price...then I just siliconed the underside of them so they wouldn't leak...worked great for the time my wing was off...
To bad I didnt have a 99, I would use up that idea. What are those Xmas tree plugs? are you talking about normal clips, that once there in theres no taking them out? Like door panel clips?
they are similar to the door panel clips, but the ones I got are like the ones that hold the kick panel on(down where the ECC is), and the x-mas tree part of the plug was slightly smaller than the hole was so all I had to do was silicone them in and when I wanted them out I just pushed them out from the bottom and they came right out...