First, it's interesting to read this:
Seems they want to do mandatory searches and no weapons are allowed.
On the main Canyon County website, it lists a county fair commissioner on it's contact page.
I sent an email to the county asking about it's status as county property. I will let you know what I hear in return. If it is indeed county property, you don't need to submit to any unwarranted search nor should you be required to give up your gun. Good luck.
Just for review, here is the email/response I received back from Ada County when we thought it was going to be an issue at their fairgrounds. Mr Patrick Calley's response might come in handy when you write the Canyon County people to inquire about their illegal sign-age and enforcement of personal preferences instead of the law they were hired to defend.