Best Canon EOS body for FILM under $300?

If you mean OM lenses then an OM to EOS adapter will do the job. I bought the cheap version and it worked just fine.
If you mean the OM magnifier then you just remove the rubber eyepiece and you just slide it in place.

 

I meant the magnifier. I didn't know it fits. Canon's equivalent is much more expensive and I already have OM body and lenses anyway, so I was tempted to buy one. I also have a Canon digital camera and use OM lenses with an adapter. Critical focus can be tricky, especially with telephoto lenses so this could help.

 

The OM magnifier has proven very versatile. For instance my Pentax K and M bodies require 2 different magnifiers but the OM product fits both. I believe it fits other brands too.