I probably should have talked more about how to use it in the post.
I tie a knot in each bundle, which makes it very difficult to pull the bundle through the slit in the EVA foam. It wastes a little bit of floss that gets cut off at the end but, seems worth it.
It will hold un-knotted floss, just less firmly.
One could likely also tie a knot that could be easily untied (e.g., a half bow) and, get enough hold.
What is required depends on how gentle you are with your floss separating. You still have to be fairly gentle anyway. Application of too much force will likely get you tangles, even with the ends separated.
I separated my floss without doing so but, I will likely clamp it to a table next time.
A variant where the end of the floss wraps around a post (with an additional slot) or through a dual-slot configuration might be an advancement.
This was an expedient solution with little planning and no iteration. It worked well enough that I thought I would share it.