Having made up the wheel blanks from two pieces of ply glued together I thought that if I made a large enough jig that was the same width as the wheels - into which I could insert a round file that was a nice fit to the jig - and I could also sit the blank - I would have a good chance of making a grove in the centre of the blanks by hand with out using any additional equipment.

With the file in the jig I held them in one hand and with the other hand held the blank. I rubbed the blank along the file turning the blank as progress was made until after about 30 mins work the necessary groves were in place on both units.

The only improvement that I can now see would be to have the jig so that it covered the whole length of the file.  I bought the file from B&Q, especially for the purpose as it was such a good fit to the jig.  It is their 200mm Round File with soft grip handle.