Red is about 655 nm wavelength. So you will need to find a detector that can sense light in that range. Then most likely you will need to have a circuit with an op-amp to amplify the signal and the output from the op-amp will be your relay activation signal. 300 meter is a really long way. Are you doing this on an open field? The best thing to do is use a 1000 feet long fiber. Just give me a little more info and I might be able to help you out.
What's the source of the laser light?