I've made bags for lots of different medical equipment, but not a defibrillator. My general approach is to keep it very simple, and basically use a standard bag or purse pattern, only customizing what needs to be changed. I never use complicated fastenings or ties as they can be downright dangerous when time is of the essence for access -- that's where velcro is a good thing. And in my experience, make one out of cheap muslin or prequilted fabric first to work out the kinks before using good fabrics.