have made quite a few knives and I thought this old style lock might work for the one you have made.
Get a small piece of flat strap and bend it to a staple-like shape (see "Back View") and drill holes at the ends of the strap where it would attach to the handle.
it utilizes the rivet hole you have and only requires that you use a bandsaw or hacksaw to cut a "lock" cut in the handle and picks while in the open position.
by adding a new hole for your tension key, your new lock effectively "locks" the piece in place when not in use, so you dont lose it as easily.
(as you can tell I like to play on the photoediting software 
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