It took a couple visits to the site to figure out how to become a sponsor. I did not know merely donating was how you became a sponsor. I don't have any ideas on how to improve the process or how to make it easier to figure out, but I figured you would like an opinion on it.
Edit: You could possibly change your message above the donation box that reads
Please help support S4GRU with a PayPal donation.
To include that you become a sponsor by doing so. Just a thought.