Thread Size for Axle Nut.

Generic steel rims or Toyota, say Corolla? With the latter I push on a black 2” ABS pipe End Cap. It’s slightly loose, so I fashioned rubber bands out of an bicycle innertube, push them onto the raised hub opening flange, then tap on the cap. Used them for about 6~7 winters now, never lost one.

BTW, I would clean the hubs and coat with some sort of rust preventative, say fluid film. My pref is boiled linseed oil. Also, if using open-end lug nuts, putting a drop of light oil on the tip of each wheel lug stud stops rust bloom.

If you think about it: threaded covers would only work on the front, btw.