I tried the same technique and was disappointed with the results. I've since repacked the Robin Hood silencer with a product called Acoustafil. The website is found here:- http://www.saveguard.com/exhausts/acoustafil2.htm
You will find it on EBay. So far I'm quite pleased with the result for such a low cost option, although a decent silencer is the real solution.
Cheers, Graham