The first thing you need to do is leave the job you are in now, if the boss is so tight fisted and has so little regard for you that he won’t let you run the engine to cool it then he doesn’t deserve to have any drivers. Second thing is don’t run the engine at all for anything, all the heat off it is soaking into your cab and heating it up. As soon as you get parked up get the grille up to let the heat escape while the engine cools, keep the curtains closed too. I never bothered but a few of the lads at my old place swore by JML chillmax air conditioning units that you charge up and add water too, apparently it can get the cab so cold you have to turn it off during the night, only £30 from Asda too