Originally Posted by BCJ
...after thwart removal, there is nothing to hold the tubes out where they belong from side to side...
The frame does that. Unless you are referring to a bit of a smaller dimension as others have suggested, no need to jam anything in there to make up for a thwart.
I have several inches difference between the width between my tubes and the width of my dry box and my cooler. Sometimes I put something thin in there, most of the time I just leave them be. Strap them down, they are not going anywhere. Makes them easier to get in and out anyway IMO.
My dry box is a standard 38" too, and my cooler is 40" I think. 42" between the tubes as I recall. No issues, plenty rigidity with the frame, etc. And no rubbing, no risk of a dry box corner puncturing a tube. And no extra charges for custom boxes.