The exhaust of the Suzuki was just a hollow pipe when I started. I've tried a whole bunch of things to get it to sound a bit more civilized, but all that did it ruin the exhaust in the end. So I had another go at it yesterday. I cut of the end bit, welded in a slightly longer stainless steel pipe with a slightly larger diameter. I had to keep the end piece of the original exhaust, since that's exactly the size of the hole in the back. To be able to change the sound without having to cut and weld I welded in a piece of threaded stainless steel.
I started of with a few shimms in the exhaust and it already sound a LOT better. It's really easy now to adjust both the sound and the back pressure in the exhaust. I should have done this way earlier.