The phrase "a flat upper" is correct and usable in written English.
It can be used to describe something that has a flat surface on the upper part, such as a piece of furniture, clothing, or a geographical feature.
Example: "The table has a flat upper, making it perfect for placing decorative items."
Alternatives: "a level top" or "a flat surface above".
So his patented airship with sails would be shaped like a boat with a flat upper deck so that passengers and crew could mill around outside in flight.
Among other Baining groups, the best-known type of mask consisted of a flat upper panel, which was either circular or divided into two lobes, and a gaping mouth, from which hung a chinlike or tonguelike form.
Again there is a flat upper part for efficacy.
As shown in Figs. 1, 2, 3, and 5 there is for several triptans, sumatriptan, zolmitriptan, and frovatriptan, a flat upper part of the dose response curves.
A flat upper cavity surface was fabricated by pouring degassed PDMS onto a silicon wafer to a thickness of about 5 mm and baking for 4 hours at 60°C.
Cells in microwells rarely had a flat upper surface.
What I had on my foot was a penny loafer, but that's when I realized that the authentic penny loafer — with a long, flat upper stitched to vertical sides — was not what I wanted.
Subsequent freedom from attachment, as in the scallops (Pectinidae), is associated with an almost circular outline, flat upper and cup-shaped lower valves, a deep radial sculpture, and, typically, bright coloration (Pecten).
The pools formed had a distinct, but not perfectly flat upper surface.
The plant's many nutlike fruits are produced in the flat upper surface of a spongy receptacle or expanded, fleshy, capsulelike structure, which is wider at the upper end than at the base.
The town is built on the flat upper surface of a lahar deposit from Mount Rainier that was emplaced about 500 years ago.
