I second that. I have both a kenko 1.4x and 2x TC's. They do not fit on the EF-S 60mm. But they do nicely on my 100mm EF macro.
Me too. Both topside an under I always use 60 mm EF-S as is and for the range of situations and subjects it is good for. It focus too close to take benefit of close-up lens. I only use the Canon 100 mm when I can't get close enough with the 60 mm.
But When I want more magnification I only use the 100 mm with close-up lenses (+4 is my favourite) or the Kenko TC (both x1.4 and 2x) for more scary subjects.
Underwater I'm now testing the Woody's diopter with 100 mm and I must say I love it
For topside and more extrem results I'd like to give a try to normal lens reversed with and adapter ring. Maybe the kit lens could give good results reversed ...