Ted:
I had the same problem and still haven't found a permanent solution. First I cleaned the deck with alcohol and gorilla-taped the plastic cover on (I only got about 10' before it blew off without the tape). This worked fine for a couple of mowings, but then the whole cover suddenly just about disintegrated and came off in three pieces. I took it back to Home Depot (purchased through HD website while in store) and they ordered and gave me a new mulch kit. That one lasted about 1/2 a mowing on my acre and something blew a hole in it--at least the tape held!
I had a roll of flashing in the garage, so cut a piece to cover the hole, gorrilla taped everything down and mowed the lawn today. So far, so good. The bottom tape ripped, but everything held, so will retape it.
I went online and couldn't find anything resembling a metal cover for this unit. I made a couple of inquiries via email, and one response I got said to contact Rob at
http://www.rfhoutzandson.com/. He responded that Cub Cadet had a new mulch cover that he has in stock that stays on better, but it is still plastic. Am still looking for a metal cover since I'd imagine having it special made will be pricey. For now I'm hoping the flashing and the gorilla tape keep working!
Donn