I would not suggest the plastic cap.
If the cap falls off, or is bent inward toward the sensor by a jolt/drop/etc it may scratch the IR filter, which is an expensive repair.
If you opt for the Hassy metal cover just make sure you purchase from somewhere that will let you try it first, or at least return it if the fit is not good. They *should* absolutely be interchangeable, but you want the fit to be very good so that the plate is not easily knocked/rattled off.
Re: the price - I agree it's a bit silly. But I don't know the last time we "sold" a Phase One DB Plate. The plate comes with every new, refurbished, or pre-owned back that a dealer sells (as does a warranty, all new accessories like batteries/cables/chargers/etc, and dealer support). If we had one of our clients lose one we'd try to find a more creative solution for them than the list price for that component as a matter of customer service.
