Save yourself some time, hassle and perhaps a little frustration by thinking about a high visibility pack rain cover.

It may not be the slickest looking option, but some are made in high visibility colors and they offer rain protection to boot.

There are a ton of different covers made by a load of manufacturers out there. Here are a few I just took off the top of a google search:

Pack Cover

Ultra-Lite Cover

Respro

Cheap Cover

Amazon Search

CamelBak (Couldn't link to it on Camelbak's site)