Pretty cool! How hard is it to ride with it? Do you have to compensate a lot for the sidecar's drag in order to go straight?
I think that the only way to make it significantly lighter, while maintaining the strength, would be to go with aluminum frame and fabric, the way kids trailers are made. Although, that will raise the costs since aluminum is expensive. You might try buying a used kids trailer and hack it to experiment. Also, it won't have the character of the wooden one!
Adam