If you leave your dog in the van in warm weather then regardless of ventilation it’s likely to start panting.
A member of the public will see it and at some point the police/RSPCA will be called. They will crowd around the van and decide the dog is distressed (they don’t discriminate between heat stress and guarding behaviour). The van will be broken into and the dog ‘rescued’. You will face a stern talking to and a bill for damage in the best case or more likely proceedings.
A cooling solution is high risk – what if it fails?
No easy solution but in emergency I have left my dog in a cage in the shade with water locked with a small combination padlock and my mobile number fixed to the cage. I have also abandoned what I was planning on doing and gone home.