Git: ignore files but not sub-directories in a directory

I have a project in which all development is done at some directory depth, and so I'm used to cd-ing to a second-level directory and write some temporary files there. The tree looks like this:

repo
    .git/
    project/
        dev/
            important_stuff/
            more_stuff/
            README
        prod/
            important_stuff/
            more_stuff/
            README
        temp_1.txt
        test.cc

I would like to git-ignore exactly files that are directly in project/ - temp_1.txt and test.cc in this case. How can I do this?