Used to use a Tesco bag when in work. Put the lens attached to camera in first, then put the lens hood on, get some black tape and stick tape around the join of lens hood and lens, this holds bag in place, another bit of tape to stick the bag to the top of the prism just over the hot shoe mount. Job done apart from making the hole for the lens, with a small coin just run this around the edge of the filter on the front of the lens lightly, and the plastic bag bit will fall out.
Not the best I know, but when the company wouldn't buy rain protectors this is all we had.