That particular float is my biggest with the sides each being 36 inches long with a 15 inch gap at the opening for the 330. It is very heavy and larger than it needs to be. I have made 6 of them of varying sizes with the smallest having 30 inch sides with the same 15 inch opening. They all seem to work about the same. They are heavy and awkward and take up a lot of space in the boat but are pretty handy when I place them in front of my place in the spring for traveling beaver. Last spring they accounted for 15 right by my house and a few years ago when we had an early ice out, I caught 32 bever within 50 feet of my place on just 3 floats..
They have their place as another tool, but my preferred method is a foothold on a drowning rod where possible.

I use pretty much the exact same idea with styrofoam sandwiched in there.