How did you widen your trailer.?

I have a trailer now that was a popup that had been turned into a flatbed. well now im wondering since the middle rails that go from to back are only 4.6 apart how do you usually combat the issue of it not being 5ft wide with a beam under the edges or what would your suggestions be? thanks for any help given in advance. oh and the back will be stretched as well to be an end trailer of 5x10