I have done this quite a bit. Measure the speakers and then find a box that is at least 4 inches bigger in every dimension. I nomally use a dish packing box. There is a box store here that has them for 6 bucks each. They are double walled. If you scrounge you can sometimes get lucky. Look around the back of appliance stores.
Line the box with at least 2 inches of strong styrofoam. Then, depending on how well the box fits, either more styrofoam or stacked sheets of cardboard, until all of the room in the box is used up. You cannot have the speaker moving around in the box.
An alternate method that I have seen is to wrap the speaker with strong large bubble bubble wrap, at least two inches thick. Then cut four pieces of thin plywood to fit just inside the top and bottom of each of the boxes. Put a piece of plywood in the box, then the padded speaker, then the other piece of plywood on top. My JBL 240Ti were shipped to me that way, and they arrived in great condition. I normally use the styrofoam myself though.
If you want to economise, you can glue up some styrofoam corner blocks instead of lining the whole box. Make sure that there is at least two inches of styrofoem between the speaker and the box, and you will be OK.
One more tip, no matter what method you use, it is always a good idea to cut out a piece of cardboard the same size as the speaker baffle, and put it on the front before starting your packing. One extra layer of protection for the drivers.