It was actually made from Coroplast (hollow plastic sheets used for yard signs). We thought that angling the arm holes would make it easier reach inside the box. Frankly, it didn't make that much difference.
The overall design was very compact when collapsed but difficult to keep light tight.
I guess the use of sleeves with elastic cuffs from an old jacket is the idea most of us had first. But your idea with the hairband and garbage bag seems a great ad-hoc solution as even the hairbands should be available around the corner.
I need to make this or at least something similar.
I have blackout curtains in my kitchen and assumed that a changing bag would be as flexible. My first ever changing bag arrived and it is stiff like a thick tarpaulin. Doesn't seem fun to use at all.