Fake dry sump tank/breather. It will function as a breather box and be positioned in the correct spot for a Spyder dry sump tank; it is scaled-down somewhat to make room for the Accusump, with which it will be cheek-by jowl.
Thus, my car will have its behind-the-passenger area of the firewall occupied by the correct(ish) looking and functioning "oil tank and breather components," as original.
[bows deeply; runs away cackling]