For those who want to know, since it took be a while to track this down: the 5lb Harbor Freight tumbler uses a M6 wing nut to secure the lid. Finding anyone who sells Locking Wing Nuts these days is next to impossible, unless you buy a bunch and want to pay way more for shipping than you pay for the nut.

Instead, just buy a regular nylon insert M6 nut (pretty much anywhere for a few cents) and then pick up an Innovative Components Snap Lock Star Knob for M6 Size (1 3/8 diameter star). You can snap the nut right into it and have instant hand removal capability and the piece of mind that the nut won't shake loose and damage the tumbler or shoot dust everywhere.

You can get these online for relatively cheap, and you can pick them up directly from Grainger for just over a buck.

I hear you. Since I just dropped about 600 bucks on Leds, cliffys, plastic protectors, parts, hardware, paint, bondo, Sandpaper, evaporust, grease, media, polish and anything else under the sun, I had to draw the line SOMEWHERE.