Hello Mart
Yes, ventilation is most important and salads like draughts more than humans. I leave the doors open all the time on a 14 foot tunnel, which has a wooden frame with mesh in each doorway:
this was January 2010 – salads lacking light!
and the other 18 foot tunnel has four doors, of which one is mesh rather than plastic. It was two but I found enough ventilation from just one of the four doors meshed at both ends, also they do not fit very well!
these doors are made of 3 inch Douglas Fir from a local sawmill Land Logic tel. 01749 860193 for anyone in Somerset.
You could do something similar and open a corner of one side perhaps at the end where your door is closed.