Which stuffing material to increase "perceived" enlosure size?

fibre stuffing will give you a small increase in perceived box volume and will damp a reasonable amount of the back waves produced by the speaker.... just a few things to be careful of, keep the stuffing well away from the back on the driver assembly and if the box is ported keep the stuffing out of the port allowing a clear passage between the rear of the driver and the port. If the box is currently void of any stuffing you should staple stuffing to the top, bottom, rear and one side on the enclosure.
They make products--more precisely, they <i>market</i> products--specifically for stuffing cabinets. I know Parts Express has a version which I believe they call Acousti-stuff or some such term. They (naturally) claim that it's miles better than ordinary polyester. Maybe, maybe not. For all I know, it's the same material. Just be aware that it's out there.
I use ordinary polyester fiberfill here at Chez Bear.
Now, I have seen several tests that indicate that long-fiber wool is superior to polyester. It's somewhat more common in Britain than it is here. Due to the weight, some people rig ways to keep it from settling over time.