We could utilise the unglossed holde from KHOL eldamo.org - Eldamo : Primitive Elvish : KHOL

Or perhaps the equally unglossed holma, which looks like an instrumental noun (hol-ma *'shutting-tool').

+Björn Fromén I thought of that, too, the thing that stopped me was that Paul had classified the word as an adjective, and I did not check whether this was in the source or just his assumption

Ah yes hepo from KHEP. Much closer to lock than a "shutting tool" but I might use hepil

Or hempa

hempa is attested with the meaning 'cord'. To use it for 'lock' could be confusing.