Bronze wool risks electrolytic corrosion between the dissimilar metals. This can be reduced by using stainless steel pot scourers instead.
One reason for leaving open the ends may be to allow hot air to expand when welds are made in the frame. Otherwise, very high pressure would build up, blowing out the weld. Just an uneducated thought.
The obstruction in one tube may be, as in our trailer, the wiring from the hitch jack and breakaway switch threaded internally. Alternatively, mud daubers love this space.
I wouldn't block this area with foam, as this will trap moisture. Stainless pot scourers will do the trick. I have sprayed rust-proofing fluid inside these box sections through the entire length of the trailer. An important task, in my opinion.
Nick Crowhurst, Excella 25 1988, Dodge Ram 2500 Cummins Diesel. England in summer, USA in winter.
"The price of freedom is eternal maintenance."