Keep an eye on announcements, the event supervisors generally try to get access the day before and allow everyone time to practice. The facility is also large enough that they generally allow practice throughout the day alongside official flights.
The last time the national tournament was at Illinois, we used about half the space for WS, Div B AND C official flights, four or more official flights going at once, about a qtr of the space for WS practices, and another qtr for Storm the Castle with no interference or mid-airs that I remember. This year we only have WS div B and Helicopter Duration Div B? and C. Should be plenty of space for practice.