I propose using areas that are vacant during the evening (thinking of parking lots in particular) to provide proper, free, shelter for the homeless.
Having structures that are easily assembled would already help, but pieces might easily be stolen. I'm imagining having permanent, collapsible structures incorporated into the design of the parking areas. This could be in the form of walls that are stored underground and can be lifted and locked into place in the evening. One wall might have a foldaway section that could act as a bed, and another wall might have an added piece that could be fixed to act as a roof.
Communal bathroom facilities could be set up without taking up too much space or the owner of said parking area could simply ask the municipality or government to erect public restrooms nearby, in support of the idea.
While there might be some prejudice issues with assuming that the occupants might leave a mess, or loiter around during the day, these would be fair concerns for a business owner attempting to do a good deed without negatively affecting his/her business. I think this could easily be solved by involving the homeless and be a good opportunity for job creation as well. A registry could be kept of frequent occupants, and if they leave a mess or cause any trouble for the business, they could be banned from using the facilities. Doing this would also allow you to set up lockers for frequent occupants to store some of their possessions in, they would be able to get a key to their personal locker after being registered on the system. Some could even be hired in supervisory and security roles, giving them an opportunity to earn an income while also maintaining the area. I believe that the majority would value the comforts afforded to them by the opportunity enough to make sure that no troublemakers spoil it for the rest.
There might even be a security benefit for companies who do this, as there would always be people around, removing the opportunity for criminal activities while the place is abandoned.