While thinking about what a minimum safe shelter would consist of, I reflected of my time on the streets, and remembered something; culverts are a great place to sleep, especially with a 'mattress' of leaves/etc. They are sturdy, small enough to hold some heat in (with cardboard blocking the ends), and fairly secure, in that it is difficult to attack someone in one.
From there, I came up with the basic idea: a 2m section of (75cm? 1m?) plastic pipe - the kind used for sewers/etc. - with one end sealed, and a door at the other with an almost smooth pull (so attackers can't get a solid grip) and a sliding bolt on the inside. Bolted to railroad ties/concrete blocks/etc., they could be deployed almost anywhere, giving those who need them a safe place to rest.
Those I've talked to about the idea are all 100% for it, especially the homeless. I have more developed ideas involving this basic concept, right up to 'The Shire', which involves even things like electronics and permaculture that I'll post later. Ciao!