sheaf
穧 centers on a bound bundle of harvested grain stalks: a sheaf. It is a specific agricultural term for gathered and tied grain.
穧 combines 禾 (grain) with 齊, which likely contributes the sound and may relate to the idea of evenness or alignment, as in grain stalks gathered evenly into a sheaf.
The grain radical 禾 on the left and the even, aligned shape 齊 on the right suggest grain stalks neatly gathered and tied together into a sheaf.
For セイ, imagine saying 'say' as you point to a sheaf of grain: 'say' -> セイ, and the sheaf is the answer.