Adding the wing tube sleeves is next. To increase strength, we decided to make the plywood rib doublers longer and use them to connect the wingtube and main spar.
This way we can transfer the loads from the main spar to the wingtube through the hard plywood doublers, rather than relying on the soft balsa ribs.

The root rib is made of 3mm liteply, so it's strong enough and only has a small doubler for local reinforcement.