The Canon converters won't work with those lenses because the front
element sticks out about 12 mm.  I know the Kenko works, probably the
Sigma ...

I would recommend the Kenko Pro 300, or Tamron SP, which are both the
same and almost as good as the Canon.  You can't use the Canon one, most
likely, because it only works with Canon "L" telephotos.

Also, I don't know the lenses you are working with, but if they are not
very sharp, a TC is not going to do anything but make you lose light.
There's no free lunch with TCs; the main lens has to be sharp, and you
are going to need faster shutter speeds and wider apertures and/or
higher ISOs.  Usually, direct sunlight is needed (or a strong flash).