Dont forget that most tapes are not waterresistant and will come off. Velcro tape would be much better.
Im protecting my camare with LensCoat Bodyguard: http://www.lenscoat.com/bodyguard-clear-back
And one OpTech RAINSLEEVE: http://optechusa.com/rainsleeve.html
The Rainsleeve comes first. It has a hole for the viewfinder wich i will fit arround it and then put the eyepiece back on for sealing. The below part where you can put you hand it simply gets folded together (i cut mine a little shorter even, but long enough so you can fold it over the tripod mount). Then i fit the Bodyguard over it, so the Rainsleeve fits really tight arround the camera preventing condensation. As carrying strap i have a sunsniper in the tripod mount, this will go through all the protective layers and seal them (made a clean hole in the bodyguard for it, i also sew the openings for the regular carrying strap shut)
The Bodygaurd is not really for water protection. But for light rain and a short while it protects you camera very well too. If you dont need to worry about the lens. For the heavy stuff, put the rainsleeve under it.
I was shooting a waterfight for 3 hours with this combination and no one spared me ("hey his camera is protected, get him"). It worked perfectly with a water sensitive 5D MII and a "i dont care if it gets a few drops" 70-200 2.8 II. Even got some water loads directly and the lens while holding it to shoot.
Of course i dont know if you can get that in time. But a plastic back with some velcro tape will do the same. If you find something to fit it tighly arround the camera (the 50D is probably not that sensitive as the 5D MII, but its better to be on the safe side).