Friendly Hiker,
You can usually find spaces in the hiker sites, which are situated between the overflow parking lot and the main parking area opposite the portal store.
There are 12-15 of these sites, and they are on a first come, first served basis ($8/night?). The sites are not fancy, but there is a restroom, water and bear boxes nearby. I have never seen this area completely full.
Hope this helps.
Mike A.