I use a lizard skins neoprene shock boot, but cut in half as I only want to protect the exposed shaft, not the can body, and that way I get two for the price of one!
However, I do only ride in dry and dusty conditions, I have no idea how it would stand up to wet and muddy.
I did worry that it would have the opposite effect to what I wanted, and would just trap grit and rub it against the shaft (oo err missus!). But, it seems to keep the dust out very effectively. I do wash and dry it and re-lube the shaft every couple of rides tho’, so its not a maintenance free solution. But, there’s no scoring on my shaft after a couple of years use, and the Giant rear shocks are very badly placed for copping sand and grit thrown up from the front wheel.