There always seems to be a little room left in the frame when you put the poster in. I just find some colorful fabric to help fill in the white. Here are some of the frames ive put together...
This is my favorite. Its a grateful dead bandana ive put behind the poster.
This one I just found some fabric and made a little sun, grass, and water colors for the background.
This was a birthday gift from a friend last year. They got the poster in the lot its not officia and took one of my tie dyed towels and made it for me