Putting another axle under this trailer is more than I want to spend or do right now . Has anyone seen a mechanical brake where there are pads riding against the tire , I know its a redneck rigging but very cheap and I think pretty easy to do . Run a tube under the trailer that comes out in front of the wheels , weld it solid , then run another pipe inside it that goes past the other tube and weld steel pads on it , put a lever underneath in the middle on the pipe that moves and run a cable out to the tongue , maybe put a boat winch on the under side with handle on top and give it a crank to tighten it up and spring to pull off.
That should work. Look up images of horse drawn wagons, that is the type of brake system they used. Newer wagons still use the technology, and use standard car tires now so there may be a supply store for what you need.
If not once you weld up the "backing plate" where your pad would be, cruise your local interstate, and find a tossed retread from a semi. I have one I plan to use for tread sections on my ramps to load my mower, atv, ???
Anyway once you have the tire tread, cut it to fit the backing plaate, then get you some counter-sunk bolts, run though the tread into the the backing plate. No more use than it will see (only 2 hills per trip), it should last a while, and the rubber to rubber contact should provide good friction w/out a lot of force, as this will be manually activated.
You will probably need to bevel the leading edge so the tire cannot get good purchase on the edge and pull the shoe off the backing plate.
Once you have all this done, add a way to lock it in place, and you know have a parking brake as well.
dw