Frank,
In general, you don't. But you could, which will take a bit of fabrication. It depends on what you have the tools to accomplish.
One of the Cox-type mufflers used a steel "washer" and a machined aluminum "bowl." If your Webra has a small shoulder between the finned cylinder and the aluminum case (I think the Mark II does) you could register a close-fit steel "washer" there. A section of tubing could form the outer diameter. A press-fit aluminum slug could close out the tube. Drill a hole for the sleeve to poke out.... Etc.
To avoid taking apart the engine every time you want to install/remove the muffler, you could make it like a clamshell--in two halves. Then your hose clamp idea could be used.
Another way to deal with this is to put it into a cowling, not on a profile. Then put a collector groove in the cowling and line it with an thin aluminum, probably J.B. Welded. A diesel exhaust is hot enough to char epoxy coated wood, so you need some kind of metal lining wherever you direct the exhaust.
Just "hiding" the exhaust helps. Imagine on a full fuselage plane that the cylinder is enclosed left and right and there is no direct path outwards towards either wingtip. Put a top (bottom) on it so only the tommy bar sticks thru. Can still be a wide-open channel front to back but it will knock down some of the bark.
A .15 sport diesel may not be too loud. A .15 racing diesel makes a lot of noise. And noise level is always in the ear of the beholder....
Dave