Hi Agrimes, here is the solution:
You need a new "Sealing Gasket Ring" for your Eheim filter. From your description I'm pretty sure you need Eheim part no. 7343150
This would fit models 2026, 2126, 2028, 2128, 2226, 2326, 2228, 2328, 2227, 2327, 2229 and 2329 so should fit your Eheim Pro II no...