Here’s what I came up with. Instead of grinding down one of my expensive F clamps, I found a large C clamp I’d be willing to part with.
The seat on the screw of the clamp wasn’t large enough to mate with the valves nicely, so I used a hole saw for the plate steel, then welded it to the seat of the screw on the clamp.
Let’s see if it works, can’t be worse than the POS that clamps to the valve spring.