Here is what you need to make your own:
Step 1 - Buy film, the kind you would put in "film" cameras. (Ask your parents what those are if you have no idea what I'm talking about.)
Step 2 - Place the roll of film in your front pocket while still in its plastic canister.
Step 3 - Develop the film if you suspect you have been exposed to radiation.
That is it! The film canister protects the film from visible light, but it does not protect the film from many types of radiation. In our work place, we tested various types of films and film speeds. We exposed them to various amounts of radiation. To sum up the results, the fastest films would begin to show signs of exposure at around 20millisievert equivalent exposure to an average sized human. The slower films took 100-60millisievert. These are very rough estimates at best! The type of radiation matters too! While these levels would not cause serious harm to a human, they do notify the person that they are in an environment that is not radioactively normal. If you are looking for a truly DIY system, you can develop the film yourself in a quick and dirty dark room. The beauty of developing the film yourself is that you don't need to develop the entire roll. You can develop the film a few frames a day, and then compare the images to see if the level of your exposure is drastically changing over time. I won't get into the details of building your own darkroom as there are many guides already available online.
So now you have developed your film and you see nothing. What you want to have is a baseline image to compare to, otherwise you don't know what "normal" is. So either develop the first frame of your roll when you first get it, or develop an entire roll if you are using commercial development.
Here is what you are looking for in the developed images:
1. A general gray, yellow, or white color across the entire image.
2. Noise/grain on the image, either across the entire image or in patterns and spots.
3. Gray or white streaks.
4. A faint image of your ribs. =)
Remember to compare the image to the baseline image. It is also important to develop the images using the same conditions each time. You should always develop at least three frames from the roll for each test. Develop each frame separately to make sure human error in one of the developments doesn't ruin the entire batch. What you are looking for is at least two frames that shows exposure to radiation out of the three. A single frame out of three showing exposure could be a fluke and should be investigated by developing more frames.
If you are looking for a way to test your system, check out this article on how to make your own X-ray source from Scotch tape: http://www.wired.com/wiredscience/2008/10/video-the-scotc/
So that is it. Place film in your pocket. Develop a baseline. Compare images to the baseline to detect a sudden increase in exposure. Hopefully, no one will need anything like this, and the reactor in Japan will be shutdown safely. However, knowledge never hurt anyone.
Disclaimer: In no way shape or form do I suggest you use a system like this as your only method of detecting exposure. This will give the wearer only a general idea that they have been exposed to radiation. The results can easily be compromised by light leaking into your system. If you think you have been exposed to dangerous levels of radiation, seek medical attention immediately. Do not bother developing film in your basement.