I've DIY'd three pull up bars over the years:
- Three foot length of galvanized steel pipe, thread a length of rope through it and tie either end of the rope securely to rafters/tree branches outside. Disadvantage: when you're doing pullups it kinda sways back and forth a bit, which I found quite annoying (especially for weighted pullups).
- Cut two lengths of 2x4 about two foot long. Drill a hole in each piece of 2x4 that's the same size as the length of pipe. Stick the pipe in between the pieces of wood, and then screw everything to basement rafters. Worked pretty well, but eventually it developed a little wobble.
- Same setup as #2, but I added several additional pieces of scrap lumber to better stabilize the bar.
A pull-up bar is pretty simple and cheap to make, you just need to look at the area you've got and figure out a way to hang it.