Had a roughly tshirt sized one with a thick plasticky outer and thinner cotton like inner that zipped open at the bottom which was fine but the thin inner material sometimes droops down and gets in your way.
Got given another one with an enlarger recently, this one is about the same size, maybe a bit bigger and still double layered but this time the inner layer is stiffer like the outer so it doesn't droop. This one also has no zip, you load though one of the arms with is wider than the other.
So, in short, maybe go for one with a stiff inner.