we glued a piece of styrene to the inside of the hatch but have also cut some larger corflute which then sits on 2 spring loaded curtain rails within the hatch so it sits just above ceiling height - styrene is permanent while the corflute is easily removed to allow for ventilation

Warning, all vans are required to have permanent ventilation. The bottom vent is usually in the bottom of the entry door. The top vent is required to be remote from the bottom vent.

Does your van have a permanent ventilator on the wall opposite the door and sown the opposite end. Most vans with Four Seasons hatches do not, the top vent is in the hatch. See this link. you will see the following under the heading "Features."

• 161 square cm of fixed ventilation, even when closed.
• Complies with gas code

If your van does not have the necessary wall vent then do not block the ventilation in the Four Seasons hatch, it is necessary for the gas regulations.