Typically bermuda is the one grass that will find a crack to grow through if there is one. Therefore, when you are building your raised bed, extend the ground cover that you use below the sides of the raised bed. For example, if you use a carboard, extend the bottom below the walls and use anoth strip along the sides to prevent soil from seeping out the walls (if that is a possibility). Even bermuda will eventually die if robbed of sunlight, but it takes longer than other grasses.

Last year was my first year at a sfg, and i placed my garden directly on top of the bermuda, and placed newspaper (about 3 sheets thick) along the bottom. I just knew I would have grass problems, but it was very minimal. It was just along the edge of one side, and it would grow out of the garden between 2 boards. It was fairly easy to pull as the soil is so loose...as opposed to pulling it out of your flower beds. I agree with the other poster that if you can line the walls of your bed, that would improve things. But Bermuda finds anyway it can to get it...so maybe just try to keep it weeded out. My bed was 4x8 with one half of that being twice as tall as the other (2 squares together, with one being tall) and it was along the short end of the tall side where the grass grew...and it would come out between the boards (since this was taller, I had stacked the boards if that makes sense,) I really thought the grass below the paper would make it's way up, but that didn't seem to be the case...it was the grass along the outer sides that made it's way in I think.