Get a piece of gal pipe about 4 inches in diameter by 2 feet 1/2 in length and let it into the ground just below ground level with concrete all around it down lower. Just dig the hole one shovel wide the wholeway down so you only need one large bag of pre mixed.
Get another length of tubing that slots into the inside of the first pipe;get one about ten feet long. (yeah,like a give way sign pole)
Get it bent over on the top so you can hang your bag on it when ever you like and take the whole lot away when ever you like too.Just tie a cover around it or take it inside when not in use. You can just lift the whole thing out of the ground if you ever move and all you have to do is get another short length and a bag of quick set and your away again at the next place. Drill a hole through the end of the long upside down L shaped one and put a bolt through it so you can put a hook on it for your O on the chains then you jiust lift it on and off.You may need to drill through the base of them both and put a bolt in it to stop it twisting but thats all. It twisting around can be ok and good pracitice for shifting targets.