A lot to be said about material...

Yes balsa density is import but, to get really light, you have to get creative. For example I cut ribs from foam. Simply cut a core and slice them off. You have to go a little thicker (how thick depends on density of foam used) than the original balsa rib but, still a nice weight saving.

Less glue (I use thinned aliphatic quite often), tight joints (less glue) and, as already mentioned, lots of sanding. :thumbup:

Composites, honeycomb structures balsa/foam hybrid construction etc. add a whole new realm of possibilities, the key is experimentation and innovation.

my 2cents.