Hi, I haven't personally experienced an oval vent for a gas appliance. However, just google "oval B vent cap" like I did and you will see what it is. They are pricey.

Or, trace the outline of the oval with paper and a pencil, and take it to a local flashing fabricator so they can fabricate something for you.

Depending on the size needed, I saw several available online in the $50-$60 range. I wonder if OP is likely to find something for a comparable price getting one custom made

Oval vent pipe are typically used to conceal a water heater or heater vent in a wall. Measure carefully like @kudzu9 suggested. It may be Amerivent, Duravent, American Metal Products.

Pro tip - paint that flue to blend in with the roof