You're not wrong about the problems. What a day I had ........
Here are some pictures of my attempts. (I am not very artistic in the human modelling dept) but you should get the 'gist' by the sizes shown.
First I made the man and was governed by the height allowed by the seat and roof measurements. He looks a bit chunky but it's the height and size we were after.
Several points jumped out at me, notably the size of the oar handle was a little large and the man was getting smaller as we progressed. we are going to need a pivoting waist and pivoting elbow joints. I don't think we need legs but I suppose it would look better with them.
I did find a smaller commercial 'soldier' which I re-shaped with the hot air gun, but his features turned out to look too small.
Here's some pictures. According to my calculations he would appear to 1/42 scale.
It will be 'mechanism time' soon so buckle up and start your engines (as they say)