Beater bike is the operable word here. My suggestion:

Assemble and tighten properly. Drill a 1/8" hole thru the nut and steerer, then tap for 6-32 all the way through nut and steerer. Install a 6 - 32 cap screw with a little blue loctite close to the head. Done.

You could tap just the nut and use a cup point set screw as suggested above, but it will mar the steerer thread. Probably about as good as using Loctite on the steerer threads. If you use one of those methods be sure to file marks on the nut and top of steerer so you can easially see if any any radial movement has occured.

:-) Jtfind