Having made up the box for the rowing model yesterday, today I've been measuring and modifying. I've made the main box just over 15mm lower than before whilst keeping the crank holes in the same height but spacing them further apart.

The spacing is to allow more room for three rowers to fit into place whilst the reduced height should make the oars move further back and forth once they are connected. 

I'm rather pleased with this part. This single piece folds up to make the dog-leg crank holder shown below. Woof!

There are two of these pieces which link the three cranks together. I've added the folded over inverted T pieces to the bottom. These should give rigidity to the part (to stop it flexing) as well as being small enough not to catch on anything they shouldn't.