Are you performing the compass calibration at these locations you're getting compass errors? If you have a clean compass cal from an area that is not giving you errors you should be able to start the phantom and takeoff from the locations you mentioned. What I'm saying is once you have a clean compass lock you don't have to do it again in an area that has metal objects. Just make sure to read the compass values and make sure it's within the proper range in the high metal areas.
FYI, I fly at montrose beach, northerly island, and other spots along the lake often without any compass errors.
I can't wait for warm weather to arrive. Chicago in the summer is a great place to be at. I'm hoping we can get a group together to go out and fly.