This word had a fairly lengthy history in Tolkien’s writings. The Qenya Lexicon of the 1910s had ᴱQ. sat (sapt-) “pit” under the early root ᴱ√SAPA “dig, excavate” (QL/82). Drafts of the first version of Oilima Markirya had ᴱQ. sapsa or sapta “pit, grave” (PE16/75), appearing in its allative form sapsanta “into the grave” in the final version of the poem (MC/221). The Declension of Nouns (DN) from the early 1930s had ᴹQ. sat (sap-) “pit” (PE21/19, 24).
The Outline of Phonology (OP2) from the 1950s had {✶sapmā >>} ✶sapnā > samna “delved hole, pit”, but the section where it appeared was rejected when Tolkien revised the phonetic development of pn (PE19/85-86 note #79). In the new system, the result would once again be Q. ✱sapta, but Tolkien did not include this example in the revised sections.
Neo-Quenya: For purposes of Neo-Quenya, I would use ᴺQ. sapta “(delved) hole, pit”, with the additional meaning “grave” from the 1920s, but would assume it was derived from ✱saptā to avoid questions about its phonetic development.
samna (þ) noun "wooden post" (STAB)