There was another thread recently which showed some vents from a horse box which were installed into the rear doors.
The airflow from my side opening window in combination with the Maxxair is plenty even on a hot day but I wouldn't leave the van unattended with the window open for obvious security reasons.
However, the horse box vents would probably be OK.
Couldn't find the thread - but found the YouTube video which shows them.
Skip to 21:20 for the vents.
The other thing to consider are those louvred vents which you can insert at the top of the driver/passenger windows up front. Ventilation without actually having an open window. I plan to make some one day as they do seem fairly pricey for what they are but I've still got lots of other things to finish building first.