Bracing the roof for A/C unit

[Yoti]

I own a 92 skamper pop up. I recently got my hands on a Duo-Therm A/C unit. To install I need to brace the roof from the interior. My problem is I can't find where to purchase the Braces. I could make them but I need to know how thick the braces need to be. I understand where to place them and how to secure the braces. Can anybody help me with this?