I think this thread kind of misses the point. No matter how secure the area you are in, the zombies will find you eventually. They’ll smell you out. They may not be able to get in, but they’ll find you. Sit on the roof and pick them off for sport? Okay…then other zombies will find you. Sooner or later you’ll run out of ammo or patience. Fun for a while, granted… but how many of those little buggers ya gonna clip before it gets old?
My point is that it’s more important to have a well stocked and inpenetrable vehicle to escape in. Pick a reasonably secure and isolated place. Park your vehicle, settle in, and wait. Make sure you have unfettered access to the vehicle; I suggest an attached garage with a remote opener. Ride exercise bikes to keep in shape (you’ll see why). When they show up, just leave, carefully making sure that no zombies get through your fence on the way out. Find another place and repeat. When it’s time to leave again, go back to the first place. The zombies will be gone. No humans there, remember? They don’t eat regular food and have no interests other than eating humans, so it’s not like they’ll ransack the place. Once they no longer smell humans, they’ll leave. True, they’ll find you again…slowly. Then you move again. Granted, you might need more than 2 places to seesaw between, if for no other reason than if something happens to one (or strangers move in) you’ve got plenty of spares. The point is that you’ll always be assured of a well-stocked familiar environment, kinda like snowbirds. No need to subject yourself to a sheltered, claustrophobic environment. And remember: a super secure place means that if they do find a way in, you may be trapped inside and unable to get out. Mobility is more important than “security”, IMO.
Misanthropic as I am, I don’t agree survivors are really a problem. Assuming that most of society is dead or zombified, there should be enough provisions for all of the survivors for several years until the zombies die. And even the mean tough guys will be smart enough to know: if someone survived, it’s because they have a gun and know how to use it. They’ll approach everyone, even little kids, with appropriate caution. Therefore, you should try to form packs with other survivors (you could take zombie watching shifts). Everytime you move, you stop off to stock up. I like the Costco idea for that. I think it also pays to have multiple vehicles in case of a breakdown; at the very least, you should get some extra vehicles for planting at convenient locations with the keys in them so that if your vehicle breaks down you make a run for it. Better yet, you should have bike carriers on your vehicle so that you can ride away (you have been keeping up with the exercise bike, right?).
BTW, I know what you’re thinking: everytime you leave a location you’ll have to burst through a phalanx of starving zombies, thus damaging your vehicle and placing you at risk. A little dramatic, don’t you think? No need for any sort of high speed mow-downs, fun as it sounds. Just run 'em over at low speeds. They’ll fall over like a row of dominoes. Use 4WD if you have to.
Finally, if you are gonna go the prison route I suggest a small juvenile detention facility: razor wire and tall fence on the outside, but more livable on the inside. Ya really wanna live in an adult prison?
As a bonus, the island has an airport. It’ll almost certainly be surrounded by a damn big fence and some men with weapons to kill possible zombies (or looters), but I’ll know them all so I can clear you through.