Hello, want to secure a 50lb. home theater sub in the back of my S2K, has anybody done this? Any suggestions?

I just got the cargo net, but the hooks and the net itself don't seem very strong. Would I have to worry about the thing slamming into the trunk roof or trunk sides if it got loose, or would a 50lb. sub tend to keep itself weighted down? I guess this is a physics question as well...

Thanks!

SH

P.S. If anybody is curious, the sub is a Sunfire Signature and the thing is like an earthquake! I'm going to have to get an inverter installed to go from DC to AC, but it should be sweet otherwise!