Hi Adam,
Congatulations on your RS Aero!
The RS Aero's deck mast pot is oval shaped and it is a lose fit, your measurements sound about right. The oval shape allows the mast to turn freely and enables auto raking - the mast rakes back upwind and goes forward on the run.
All RS Aeros since #17--, and some between #1340 and #17-- have black plastic inserts into the deck mold to save wear on the gelcoat, but the dimension of the pot is the same. Earlier RS Aeros with white gelcoat deck pot tops are able to purchase the deck ring (mast pot collar protector) retro fit option, otherwise a viable alternative is simply to re-gelcoat the top rim of the pot periodically.