I also used some cheap, short (12"?) glass pack muffler on the tailpipes of a convertible that had some loud mufflers. That worked very well for me. I put them in just before the tips, kind of where a resonator would be. They were still working well for me until I replaced the entire system.
You could also add a Hemholtz resonator to the system. That shouldn't be a lot of $$ if you can build and install them yourself. There's plenty of info on the intrawebs about them, but here's an example.
