Extender or tube for fish eye lens?

i recently bought a samyang 8mm fisheye lens and it works really well with my canon 550d however i would like to get a bit of a further picture than i am getting, i know that an extender exists to magnify the picture you get but its too pricey for me, ive looked into tubes which go inbetween the lens and the camera but im not sure on the magnification of it, and i even looked into attaching my fisheye to another lens which i can zoom, apart from severe vignetting i can't get a clear picture. is there anything at all i can try to get a good zoom on my fisheye which is all manual without loss of quality, at the moment its at 8mm but i would prefer a 20-30mm