The easiest way to hold in a Lipo in an X - 6 is to flip over the stock battery brace. Check out the pic here to see what I mean:
http://www.2wdrc.com/images/TechImag...iPo_02-big.jpg
The way to mount the battery with a CF piece is to drill and tap some holes down into the 4 ejector pin pads located around the 4-cell area. You can roughly see those locations in the pic of BK's car above. I have a CF brace like this and just hold it in with four long screws/washers, screwed down through the CF and into the chassis. Brian has a system a little neater and more secure than that; he mounted some Lunsford quick-change shock mounts into the ejector pads, spaced up so the CF brace had something flat to rest on, and then used the AE thumb nuts to hold the brace on.