Most professional boxing rings have a rather springy base so that when boxers fall, they bounce against it and it's not as painful. While this is one of the most expensive parts of a professional boxing ring, it is important that a similar style is replicated in your homemade ring. Take a few used king-sized mattresses and lay them side by side to form a base. Cut thin boards to lay on top of the mattresses and fit their size. Then screw or bolt the wood to the mattresses. These mattresses with a board on top can now be covered with a tarp, creating a floor to the boxing ring that has some give when someone falls.
Posts and Ropes
By taking large PVC pipes and attaching them to the corners of the base with metal latches and screws or bolts, you can easily create corner posts, which are sturdy but not dangerously hard. By drilling holes in these corner pieces of PVC pipe, thick rope can be run through these holes and knotted at the ends. They are secured within the holes of each PVC corner pipe. The more springy the rope, the better, so long strings of bungee cord, if you can find any long enough, may be the best bet. If not, large thick rope will work, but may restrict fighters up against the ropes.
Now that the basic elements of your homemade boxing ring have been constructed, it's important to go over everything and make sure there is nothing that can seriously injure a fighter. It may be a good idea to install a layer of foam on top of the boards on the wooden base of the ring to cushion any falls even more. Using layer after layer of thick foam core tape on the PVC edges of the corner posts will keep the pipe from becoming sharp or something fighters can fall against. Other small safety adjustments will be made over time as you use your homemade boxing ring.