[Mr_I]

It is tough to answer this without actually seeing your implementation, so let me ask a few questions of you:

You say you're using "plastic bands"; do you mean something like a Zip-tie? If so, I would guess that this is acceptable.

If, however, your "plastic bands" were more along the lines of a short section of PVC Pipe (i.e., something really hard), you might want to rethink this.

In either case, my understanding of the term "hard part" refers to plywood, metal angle, etc. Good luck with the judges interpretation!