The Secondary Mirror
Making a 4.25 Inch Dobsonian Reflector Telescope
Secondary mirrors for Newtonian reflectors are elliptically shaped. From a 45 degree angle they look like a circle. These can be obtained at various sites on the web. The prices are strangely exponentially related to size. There are a few free calculator programs on the web for figuring out how big your secondary should be.
Since the mirror was small, I opted for a purchased mirror. The common wisdom is that it is harder to figure a flat mirror than a parabolic one. I'm not convinced however testing this will need to wait for a future project.