A "C" or hook spanner wrench works best for the steering stem nuts but it's best to have two, one for holding the bottom nut and a second for tightening the top nut against it. The 45-52mm size pictured below works well and is readily available off eBay for not too much money .....
https://www.ebay.com/itm/2-Pcs-Univ...rand=Unbranded&_trksid=p2047675.c100005.m1851
Now a note on the notched nuts. If you examine them, you will find that one side has a bevel around it's I.D. .....
They should be installed beveled side facing beveled side. They lock together more progressively and with more "feel" this way. This also presents the flat side of the bottom nut down to better preload the bearings and the flat side of the top nut up to better support the top triple tree.
https://www.ebay.com/itm/2-Pcs-Univ...rand=Unbranded&_trksid=p2047675.c100005.m1851
Now a note on the notched nuts. If you examine them, you will find that one side has a bevel around it's I.D. .....
They should be installed beveled side facing beveled side. They lock together more progressively and with more "feel" this way. This also presents the flat side of the bottom nut down to better preload the bearings and the flat side of the top nut up to better support the top triple tree.