We've used red clover for this purpose more than once, in a similar-sized area.

Plant in September or as soon as other plantings are removed, you need clover inches high in order to survive some winters.

Spade it under in late March or April. This is a fairly coarse ground cover, it needs several (3-5) weeks to decompose.

Grass clippings are fine, watch out for too many grass seeds being introduced.

I like to use buckwheat. Grows fast, good thick cover. Winter kills in my area, which is good, but the dead plant provides protection for the ground during the winter.