thermostat leak

I replaced the themostat (82 300td). There is leakage around the 3 bolt thermostat cover. I had replaced the rubber gasket with new. There is no gasket between the cover and the housing. The surfaces were cleaned before re-attaching. Should I fashion a gasket. It looks like the rubber one (fits around the t-stat) should provide leak protection, but it doesn't.

If anyone has had and fixed such a problem I appreciate a reommendation.