If you are still working on a design, perhaps look at what this company has to offer (at least it was something I was thinking of):
http://www.stockcap.com/prod_detail.asp?id=126&cat=14
They have a whole assortment of vinyl and LDPE caps, plugs and flange covers that may work really well. The caps themselves are generally very inexpensive ($.01 to ~$0.1 per cap) and usually ready to ship. This option isn't really available to a single user b/c they have min orders of several hundred (depending on cap) but may work well for you as a distributor.
You can get ~free samples as well to try a few differnt ones out. The vinyl are particularly nice as they are stretchy and would snug fit the motor. Something like SR-1525 or SR-1650 may work quite well for a rear cover. An LDPE cap could be found for the front if needed.
The only consideration is how well the vinyl would continue to work on a hot motor.