If there is nothing available at the Portal, try Horseshoe Meadow. There are 3 campgrounds and all are first come-first serve. There is a 1 night limit, but I've never heard about anyone being asked to leave after 1 night. Being a holiday weekend, it might be different.
The Horseshoe Meadow campgrounds are "walk-in". You will have to carry your stuff to the campsite. Shared bare boxes and fire rings, water and pit toilets. I've stayed there multiple nights without a problem.
