I once made a dummy out of a fence post, too. I went to a local furniture shop and asked for some of their scraps for the arms. When they found out what I was doing, one of the guys offered to mill the short pieces down a bit. That saved me quite a bit of rasping and sanding, but they didn't do any of the finishing for me. I didn't ask, iether. By the way, I do not advise using concrete to set a dummy in the ground if you rent.
I also made a forearm conditioning tool out of a ladder once, kind of like a four sided makiwara. I'm a little bit insane, though. Around the time I was making the forearm "thingy", my mother noticed I was missing a fingernail. She asked me how it happened. For about 3 weeks, I had been spearhanding the ice field behind our house trying to pull out a chunk of the frozen dirt under the ice and snow. I had been succeeding for about three days, but it cost me a figernail, and most of my fingernails were broken to nubs. I had let one of my nails grow a little long. That's not a very good idea when doing this stuff.
Whatever works- works.