Given that the amount required is variable depending on the player(s) chosen, how about:
1. Contributions are variable, ranging from £1 to a maximum of, say, £50. For every £1 contributed, you get one entry into the draw for the seats at the end of season dinner and the signed shirt.
2. A deadline is set for contributions, after which we then ascertain which players are a) available, and b) affordable within the money received. A poll can then be run for a few days to choose the player(s) sponsored.
Thoughts?