For your Craigslist consideration, here's a Destaco (some folks hyphenate that to De-Sta-Co) model 210 vertical hold-down clamp for your fixture building efforts, $15 cash FOB Pomeroy WA.
QR Code Link to This Post
New Old Stock (NOS), I've been toting this clamp around for 20+ years for that fateful day when I'd be making widgets, and need a bench top or work table clamp to fabricate into a purposeful jig... Time to pass the torch; you can make the jig, build the widgets, or tote the clamp around.
Made in USA, not one of the flimsy Amazon.com knock-offs.
Simple, effective, adjustable. Capable of up to 600# holding force.
CAD drawings available on the manufacturer website.
I do wander over to the L/C area from time to time and could bring the clamp along, if that helps.
Thanks for looking!