Re: Trailer from scrounged shopping cart
I think you're on to something here. I also agree that with real cargo or a passenger you need to have real good brakes at least on the bike, but better yet also on the trailer. That said, I don't think it would have to be like rocket science to set up rear brakes and still be able to connect, disconnect the trailer without too much hassle.
What immediately comes to mind is the use of a pair of mountain bike type forks... something that had V brakes which for the money (scrounged) are hard to beat. The problem area is having a kind of junction at the front of the trailer where a single cable coming from the handlebar brake lever comes to a point where the single cable connects to a Y so that it becomes a dual pull to activate both V brakes. The single cable coming from the bike needs to be able to easily disconnect at the Y. I think there is such a critter out there in the bike world. If not, then one needs to be fabricated.
I have a drum brake on my sidecar and it is simple and quick to remove the cable from the brake when I detach the sidecar. I just coil up the cable and hook it into my rear rack.
Something to think about in the middle of the night... what does that Y union/disconnect look like? How to make it from junk?
And why couldn't you use a shopping cart body for a sidecar?
Someday when I grow up I will probably lose interest in toys with wheels, but until then...