Make Sidebar Not Collapse?

I absolutely love GP, and just installed it on a second site, but I have one issue. When you shrink the window, it collapses the sidebar and allows the sidebar to get partially covered up before dropping to below the content. Is there any way of forcing the sidebar to not shrink below 300px? Ad networks require an "at all times" 300px width minimum.

Please tell me there is a way to do this. I do NOT want to have to change themes. GP is my favorite ever.