A piece of flat bar stock bolted twice or welded on one handle with two hitch pins on the other handle for quick operation with no tools. Locking both bars horizontal
I like the idea! The only thing is that the levers, when moved up from locked down to a horizontal position (pointing at each other) they pull the pin away from the ssqa frame where the pin stopper is, so the pins can be raised/pushed up past the stopper.
A flat bar solid extension on one handle connected by one bolt allowing it to pivot on the handle, and with the levers locked down, drill aligning holes at the other handle for a linch pin. Once pinned, as long as the linch and swivel pivots are a snug fit, should stop the bars rising as the add-on bar would hopefully prevent the levers lifting. That motion makes the levers just a bit closer together before going past horizontal.
I'll take some measurements of how much closer the levers are from locked down when they're both raised up to horizontal/parallel to each other. If there's an inch or more of tightening the space, that should be stoppable by a solid bar between.
Connecting the extension solidly to a lever, instead of a one-bolt swivel, will create problems when they're opened and you're trying to hook up my pallet forks - it will hit the forks frame above the ssqa and push the lever down as you try to hook it up. This already happens a bit with connecting the forks. Got used to it, but adding a solid length to a release bar will make that a real problem.