Does anybody know what 2x teleconverter would work best for my 100mm 2.8l since I can't use canon converters?
If you want this strictly for macro, you can use a 12mm extension tube as a spacer to get to 2X with a Canon teleconverter, presuming you already may own one. You lose the ability to focus beyond a short distance though.
Many off-brand converters do not have a protruding element, so there are plenty of choices... I'd tend to lean toward a decent Kenko or Sigma if I were looking, but if you plan to do a lot of photography with something like insects, consider upsizing to a 180mm Macro. The working distance alone is worth it.
It would help, though, if you could specify what your intended photographic targets are, and budget limitations (if any).