I do use a sphere. I have code that does it using an ellipsoid, but I found that the match to other FAA sources (such as visual charts and licensed digital data) is not as good. I think the FAA may use a spherical model when producing their own digital representations.

No digital source exists that perfectly matches the FAA orders. For example, the licensed data (such as is available from for-charge services) for MOAs often shows significant differences from the FAA orders. So you're correct that the only way to get a match would be to code it by hand. The orders themselves are not perfectly internally consistent either. One near me specifies radii in both nautical and statue miles, using a conversion factor of 1.1.

L.