Well you all made my night by your nice replies! THANKS!
I’ve only tested 1.3" and .75" binder clips (that is how they are referred to and also their rough measurement from end to end perpendicular to orientation of silver clip handles, see pics of calipers for clarity). I like .75" best as they seem to have the tightest grip my gnarly cut up slats in my water table. I used a few 1.3" clips and they held fine too. It may be my imagination, but it seems to me that the .75" clips have more force holding them closed…but that’s just how they seem and I don’t have any tool to verify that claim. And it seems that the fact that my bed slats are a bit beat up provides an even more grip for the clips.
This is such a creative and resourceful community that can figure things out, and at the risk of oversharing what might be obvious, I did find that .4" lip on the 1.3" clip and .2" lip on the .75" clip was plenty to hold my 22 gauge steel in place and stopped it from moving and lifting into a Pringle shape. And I left about .1" of material in the lip. In case this does not make sense, you can see my hand drawn diagram.
This is again pretty intuitive, but I buy a some of my metal at box stores and the stuff is never flat…and when using these clips, I place the metal so it is concave up (Concave Upward and Downward) so the center sits against the slats and the sides lift off the bed…then when the clips are put in place, the sides pull down and the whole sheet is flat on my slats.