I got some craft mesh, cut it to size, and stuck it between two triangular binder spines. It worked really well til my girl decided to jump it, an hour after I'd inserted it. :p I need to make it a bit bigger next time, I think.
I already had the craft mesh, and the binders cost me $6.20 AUD for a pack of 5.
Well, I understand the effects of the double dividers. I think my 5.5 is a bit limited in space for a double divider. My hood also limits where my filter goes. So I don't think double dividers will work very well.