Bolo clip plus a slider for jacket/hood cords. The cord here is cheap Tandy 3/32" lace, 4 strand round braid with no core. Just a trial experiment!
The bolo clip tears up the lace when it is slid to adjust the length. Does it on Tandy's cord too! The clamp is not easy to quickly open and close, so your clients wouldn't like that either.
The cord sliders I got were from a few years ago, from Mountain Equipment Coop (https://www.mec.ca/en/) I can't find them easily this morning. They are not wide enough for 2 cords, so have to use 2, one on each string.
Clamp open
Clamp Closed
Face for concho