Chicago is crafty about their ordinance. There is a clause for interstate transportation,
8-20-090 Interstate transportation of firearms
It shall not be a violation of this chapter if a person transporting a firearm or ammunition while engaged in interstate travel is in compliance with 18 U.S.C.A 926A. There shall be a rebuttable presumption that any person within the city for more than 24 hours is not engaged in interstate travel, and is subject to the provisions of this chapter.
The way I read that is that technically after 24hrs if you were found in possession a cop could take your firearm and fine you then you would have to go to court to get it reversed or pay the fine and lose your firearm. In practice, I imagine it would be up to the cop on site whether they even believed you had been in the city less than 24hrs, which makes it iffy to me. and I wouldn't want to find out how long it takes the CPD to return a firearm.
I was told during my CFP class, hard lockable(doesn't have to be locked) case, unloaded with the ammunition in a separate container, ie no loaded mag in the same container as the pistol. I cannot find documentation for this, in the Chicago Ordinance, it may be from Cook County or just something my CFP instructor came to believe. Either way, I'd go fully inaccessible storage for your time in Chicago/Cook County.