You can also get a bellows, the newer fancier ones from Novoflex have electronic coupling that will let you control the aperture on EF lenses.
Or besides that there's dedicated Macro lenses, like Neuro said the MP-E 65 will get you 1x to 5x magnification. Or there's other systems like the Olympus OM 20mm
that is still a very highly regarded macro lens for up to 16x magnification, that's something like framing 1x2mm. But they're expensive even second hand on ebay, $6-800, almost as much as an MP-E 65. And they won't do anything for your working distance, it's more like 2cm with them.
The only way to get closer without getting closer is Teleconverters (I think the 100L can use them, someone correct me?) then you can stay the same distance and get a 200mm f/5.6 at 2x mag...