One method I have used to hold the extentions on a totally in cowl application where I didn't want to notch the cowl for cowl removal was the use of a spring. A throttle return spring found at any autostore or hardware store has the long straight ends on them and are perfect for this application. I cut a piece of aluminum tubing with an i.d. to fit over the end of the muffler pipe, and is long enough to go outside of the cowl area. I attach the one end of the spring to the upper end of the muffler by drilling and tapping the muffler or a smal clamp around the header pipe.There are many possibilities for securing that end of the spring. Laying the sping along side of the muffler I cut the other end of the spring long enough to be reached through the cowl pipe opening with a pair of needle nose pliers. This end of the spring I bend a hook in it. Install the pipe extention, pull the spring down and hook it on the end of the extention. I use this method even if the cowls are notched for removal. That aluminum tubing polishes up real nice also.