I think you could pull this off with a little work on the shape. My bet is that if it's right off the shelf, the beret probably still has a liner in it. The first thing I'd do is cut that out, give the whole thing a good soaking in cool water and shape it to my noggin. Pick a time when you don't have anything pressing to do for awhile and wear it until it dries back out.

Now I think I'll join Skiballa and dig through my old footlocker. I've still got a few of those around too!