The cheapest, most widely recommended way to get more magnification from your 100mm macro is to use extension tubes. They contain no glass, so they don’t reduce optical quality, and longer tubes give more magnification. If you buy tubes with electronic contacts, you can usually keep metering and other camera functions.

Close-up filters/diopters are another low-cost option and are convenient because they screw onto the front of the lens and generally preserve normal lens operation. However, lower-quality macro filters can soften image quality, so extension tubes are usually the better value if image quality matters.

A key point: your 7D’s crop sensor does not increase the lens’s true magnification; it only gives a narrower field of view, making the subject appear larger in the frame.

If you want much higher-than-1:1 magnification, tubes will help but only up to a point. For extreme macro, photographers typically move to more specialized solutions such as Canon’s MP-E 65 rather than simply adding more accessories to a 100mm macro.