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Plastic bag works fine if there is one big enough. Use a trash bag if you want.
Also don't fill the alternator with water...Duh. but you already figured that. All should be fine. As I said I washed mine at the carwash and covered the airfilter. The engine was cold but I lost my PS pump cap on the way home. I must have slightly turned it and when I hit 80 or so it blew off. I bought one in a hurry at autozone and picked up my original on the side of the road the next day.
I use the garden hose on a heavy but soft mist, prewet everything then use lots of Simple green. The first cleaning is the hardest then after that its pretty easy. I cover my Tymar with a plastic seat cover we use at the shop and just wrap it around it. Then I try to steer clear of the alternator and IDM with the spray. They get wet I just don't hold the spray on them for anytime. Also If you can hot water works even better. at the shop we have a hot water spigot for the hose that we can use if needed and it does a really good job with the simple green so remove oil. If your clothes washer is in the garage or close by you can use the hot water from that.