Birmingham post your bikes/faces/places

You can put a fixed sprocket on a normal freewheel thread.

I seem to remember something about bottom bracket lockrings fitting on that thread..

...right-threaded 1.37" x 24 tpi lockring intended for old style bottom brackets.
If you are using a single-threaded freewheel-type hub, this is better than nothing, though not so secure as a real track hub with a left-threaded lockring.