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I would just use a simple 555 timer circuit, like this one:
That's probably the easiest way to do it. Only a few components are needed, and it's easy to put together. The rate of flash depends on the values of the components. I would start with a 10uF cap for C1, and 10K, 1K, and 1K for R1, R2, and R3 respectively. Run the whole thing off of a 9v battery.