Park in the visitors carpark at Sussex House (car park 4) Sussex University across the road. Its evening and holiday time so there should be spare spaces. Theres an underpass across the A27 so only about a 200m walk. https://www.google.co.uk/maps/@50.864051,-0.086826,3a,75y,118.36h,75.47t/data=!3m4!1e1!3m2!1swjXGVvwGczJRfgvj8o3i_Q!2e0?hl=en
Otherwise park at the sports centre also at Sussex University and very close to the one above. You can even see the stadium in the background of the link below
https://www.google.co.uk/maps/@50.863691,-0.089271,3a,49y,171.41h,79.78t/data=!3m4!1e1!3m2!1su6aXgCaU46iUuG1yXdhSoQ!2e0?hl=en