I guess the shorter the focal length, the higher your magnification when reversed. I reverse the 50mm as well, with less "magnificant" results
I used the kind of reverse ring that mounts onto the camera. Makes the 28mm a pretty versatile lens! Controlling the aperture lever with your finger and doing everything handheld takes some coordination, but it seems like a great payoff for a $15 adapter.
It got me my most popular shot (although that could just be because it is red :-)
Stacking onto a telephoto looks like a great idea - I'll have to look for one of those adapters.