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I just carry my triangle kit loose in the boot together with a high-vis waistcoat (I have to go on airfields occasionally). I keep oil top-up in that side strap thingy and also carry screenwash and a bag with quick detailer and cloths etc for glass cleaning and removing bird poo. I also carry a sleeping bag, self-inflatable mat and blanket - Very useful when I need to use a friend's floor for an overnight as it minimises their inconvenience and so I don't sh!t in their guest bed. I had occasion to use my blanket, mat, sleeping bag on Saturday night to assist a mobile paramedic lady, so I was very pleased I keep them in the car. It made the difference as it was extremely important to keep the patient warm - But all that is another quite long story.
I don't carry spare bulbs or first aid kit but might now consider a fire extinguisher.