I believe that I have an NDA agreement draft that you could use with maybe some small modifications. Now the key would be to find it again. I'll dig around my office and let you know. 
As far as as security - you might want to consider hiring someone that already has some type of credible security clearance. For example with myself, I hold a Government of Canada Secret security clearance that you can verify with the RCMP or PWGSC. You could ask potential candidates if they hold a similar US/Canada and/or State/Province clearance certification. It adds a little more credibility to the NDA you sign between you and the contractor. 