To the Returning Officer,
Please consider having different grades of rally spaces, e.g. L - for expected crowds of 20k or more, M - between 10k and 20k, S - a void deck would do. Also, consider charging for space NOT taken up. So if a party books a L space, and only 100 turn up, they should be charged for the 5000 (half of site capacity) less 100 not taken up.