What Timber says about stuffing and flow resistance is true, however it is possible to provide energy absorption at the enclosure walls. It just needs to be pressure dependant. This is the rationale for using foam – the foam stuffing used in commercial speakers is normally of a closed cell type so it does not provide much in terms of flow resistance anyway. It is there to provide pressure sensitive energy dissipation.
Hybrid, multi-layer pressure sensitive materials for speaker enclosures are commercially available. For example, “Black Hole” provides pressure sensitive foam layers for absorption, together with a visco-elastic layer for panel vibration damping. By using different layers of materials “Black Hole” is tuned to provide more absorption in the critical mid-band frequencies you normally find in enclosures.
I would expect egg crate foam to provide at least some pressure sensitive absorption, but my gut feel says the absorption bandwidth would be reduced compared to a composite product such as “Black Hole”. And it would probably depend on foam density and thickness..
Form the above we can deduce that the optimum damping strategy would contain both elements, i.e. flow resistance and pressure sensitive absorption.
Black Hole:
http://www.zalytron.com/ Sonic Barrier 3-layer, more or less equivalent to Black Hole,:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=260-535Acousta-Stuf:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=260-317I expect Acousta-Stuf to be a bit more effective than Dacron. Whether it's worth the trouble of importing I don't know, but it is very popular in the US. I have not used the Parts Express products, but I have used Black Hole with ordinary Dacron and that combination gave me the most effective damping. If you don't want to import, try egg crate foam together with Dacron or similar.