Re: shopping cart shelter. Interesting idea, but I'm suspicious that there would be a lot of other ways to more efficiently/expediently setup a shelter with urban materials. This does not provide an insulated area for sleeping, would not necessarily hold in a significant wind, might collect water in a rain storm if you aren't careful with the opening flap, and even though the shopping card has a wedge in its wheels, it can probably still shift, and both it and the milkcrates could probably be knocked over and compromise the whole structure. Yes, it requires less string and the ubiquitous sticks that backwoods survivalists love, and maybe takes less time/effort than assembling an good insulated cardboard box, but I'm not sure the trade off is worth it.
In November 2009 I did a night in a cardboard box for a fundraiser with "out of the cold" to raise money for local homeless shelters. Some girls in the group thought I was stupid and spent the night in a three season tent. At 2 am I was woken up by one of the girls to ask me to help them pack up their tent so they could go home, none of them were able to sleep due to how cold they were.
The next morning we had a thick frost on everything. Admittedly the box was closed on all sides, and I had two large blankets with me, and a 7 degree (celcius) sleeping bag I wasn't using. The box was from a largescreen tv, I had to curl my legs to fit in, it wasn't even shoulder width, I had to sleep on my side. In a pinch, I could do it again, would it get me through a night in a mid-Canadian winter? It would be tough, the box would likely be too warm and melt any snow that fell on it, making it soggy and lousy shelter.
Nice, basic insulation rules for the win. (Assuming your fellow fundraisers later thought about why they were cold and you weren't, and then learned something for an emergency or another fundraiser...) As for the melting cardboard, you could probably find some plastic pieces to put over top of it and keep the snow from soaking in.