Christine,
The BIG difference between Alex's method and mine are :
Alex uses a zoom attatched normally to the camera with a lens reversed onto that. The significant advantage is that you can use the zoom function to focus, and you are always viewing at open aperture.
Mine is the cheapo alternative but a lot trickier, and requires a lens with lever operated diaphragm. You have to set the lens to the taking aperture f16/f22 and then use a finger to on the lever to open the lens diaphragm for focussing. As the only zoom I now have is 28-105 it will suffer the same vignetting shown in the last pics I posted.
For your zoom you have the 75-300 or 80-400 but I do not know if you have access to a lens to reverse onto the zoom. Or are you going to do a raid on Joe's kit bag.
If you post the filter thread sizes it may help in comming up with a solution.
Don