Go to the plumbing section of your local hardware store. There you will find a wide selection of rubber o-rings, washers and grommets. You should be able something that is a snug fit over the axle locknut and axle spacers but wide enough to cover the Hyperglide locknut splines. Basically, you're creating an external seal.

I was fortunate to find a grommet that looks like it is made for this application. It snaps right into the Hypergide lockring and even has a small retention collar to prevent it from popping out. The flange compltely covers the lockring splines.