As the originator or the "Saginaw" hypothesis, allow me to offer some insight. As noted above, the bays are close to "Elliptical;" or "Ovoid", yet my detailed survey of 50,000+ bays show they conform to five stylized ovoid shapes, along with a precious few that are actually elliptical. Each shape is restricted to a portion of the annulus running 900 to 1400 km around Saginaw. I interpret the slight variations from a geometric ellipse driven by interactions between the ejecta outflow trajectory and the tangential eastward speed of the Earth's surface as one moves south from Saginaw to lower latitudes. I apply the term "skew", with an airplane crab landing in cross wind as an analogy. Here are the planforms archetypes:

My mapping of the bays uses Google Earth and the "Image overlay" facility to place one of those planforms over a bay, as a semi-transparent png file. It is the fitted to the bay in a best-effort to capture the stylistic variances seen in the archetype overlay. The overlay editing can only change the length and width of the presentation, and the only difference seen bay-to-bay is the eccentricity, While I have been offering these facilities to all for 10 years now, few have taken me up on the dare to actually look at how exquisitely robust the match is.

Perhaps it is Google Earth, or the network speeds and computer power necessary to view my hsv-shaded LiDAR-resolution digital elevation maps projected on the virtual global that has limited review by others. That bare-earth imagery really bring out the true nature of the bay rims, which typically run 360º around the bay, regardless of what has been said about them having a "prominent SE rim".

At the GSA Southeastern Section Meeting in Charleston this past March, I presented a talk announcing a web-based facility to view the bays and my overlays in a simple browser window. Even works well with latest mobile platforms. In a set of 550 web pages, I present one bay at a time in normalized horizontal 9x16 format, and it can be clicked up into full screen mode (browser ability dependent). To invite critical inspection of "my" planform fit, the web page offers a slider to roll on and off the overlay, using the visualization technique referred to as "before-after".

These 550 bays are selected out of 15,000 appearing within two USGS 1º quadrants (Florence_W and Florence_E). The link here will take you directly to the sequence of bays from largest to smallest. One step up in the page hierarchy you will find a discussion of the "bayCarolina" archetype and some examples of diving deeper into the indexing of the 550 "before-after" bays, along with links to jump into Google Earth and look at all 15,000 bays in those two quads.

Have a look: Planform Sliders, bayCarolina

(I run on a shared platform and don't pay $$$ for a dedicated IP with SSL enabled, so some browsers state "not secure", since I sell nothing and ask for nothing by way of input).