Originally posted by Toxis
I don't recommend using cardboard. 1. The only time cardboard will be in my car is when I bring product home in their boxes (hehe). 2. under moisture, it will be VERY weak and could rip and drop your speaker 3. resinates to much and will not be a sturdy support 4. it's cardboard for god's sake hehehe Just use a thin piece of MDF (1/8" or 1/4") to mount them under your dash. You could possibly even make it work with a few metal strips then get grill cloth to cover the horn body so it isn't so obvious...I love horns, but I'll stick with my Focal Tweeters.... 
Wait a tick... I made a post without being a smart ass or jack ass to someone...WOW! :fawk: :yay: :wiggle: :booty: hehehehehehhehe
Maybe I wasn't clear in my original post. I don't know of cardboard strong enough to hold up any horns.
Typically, people use heavy plumber's tape (radio backstrapping) to mount horns. At least the CD1's. It's recommended that for CD2's you think about welding in some mounting brackets (they weigh about 20lbs each)
My suggestion of cardboard was for the under dash pad only. You could make them out of carbon fiber if you wanted to. Cardboard is just cheap and easy.
The under dash pad can be basically anything semi-rigid that spans the distance from the horn body to the leading edge of the dash...hence the "attach" in quotes. Just cover it in carpet and you are good to go.
If you skip this step, you will get sound leaking up into your dash, creating multiple point sources when it leaks out....that's not pretty.
Hope thats a bit more clear now.