Easiest way to do this is a trunk swap with someone who has a trunk the same color you have with whatever wing you want. Quick plan.
Step 1: Fill out profile
Step 2: Go to your regional classifieds forum (SWIC, NWIC, SE, ect)
Step 3: Post new thread with title like... WTT: ____ color wingless trunk for WRX/STI wing trunk. (whatever wing you want)
Step 4: Watch thread and when you have an offer, get in contact with said person, meet somewhere and swap.
BTW: Either the person who you swap with or you can look up directions if you need them by searching either here or maybe on scoobymods.com for instructions on a trunk swap.