for the adventurous - structural honeycomb material - "NIDA Core" - see their web page: www.nida-core.com
probably isn't moderately priced, but if you pay yourself a very conservative $25 dollars an hour for building a trailer [include "thinking & planning time"], materials' total is probably only 35% of the project.
....and for the "bicycle tear" enthusiasts, what is 1 ounce saved peddling up hill into a head wind....?