GB The motorcycling press were less than impressed with the performance of the plunger system, and cited poor road holding as the sliders would tend to bind. Compared to the rigid frame, there was little benefit, they said, as the spring movement is small.
Substituting a steel tube for the top spring would effectively turn it into a rigid frame, and you could alter the ride height by altering the tube length. It would not clank over the bumps, but you might be going to Central Wheel a bit more often.
What makes the clank? The clank must come when the spring gets coil bound, or is it when the inner slider hits the top spring mount? Either way some sort of cushioning to restrict movement would do, say a pile of rubber washers or a slice of thick walled rubber reinforced hydraulic hose around the inner column, to match the slider OD. It fits inside the spring, cut to a length that still offers some movement, but prevents the spring binding or the inner slider hitting the spring mount. Crude, effective, cheap, looks standard. You can bet someone has already done it, necessity, mothers, invention e.t.c.
I did notice that some washers are available, not official parts, which I thought were grease seals. These may do as buffers so worth researching.
Save the anguish, just avoid the pot holes.
Swarfy