timmer... as suggested by ballastanksian .... one of the open funnels could be an ideal exhaust as it would also contribute to a venturi effect which is preferable and more effective to an open hatch or skylight
Alternatively your thought of an additional smaller fan possibly in the other chimney pushing/drawing air into the superstructure sounds fine
We can also see the smoke foggy unit just aft of the power choke board...would warm air assist or hinder the effect of this unit?
Now I am not suggesting you commence smoking cigarette's
but the only real way to confirm air direction and draw function is to use some form of smoke
You have plenty of choices...so carry on with the good work
...& keep us posted with more results & images as you progress ...... Derek