My solution would be similar....
Knock in some short sections of scaffold poles or something similar, a little distance from the base, and as deep as you can possibly get them, then lay concrete fence poles, horizontally resting against, and supported by the poles. Finally, remove those bits of decking boards, and in fill, with a sloppy concrete mix, pushing it just as far under the slab, as you can get it. As a last job, you could chop any protruding tops off the poles.