I'll show you how I made mine. I bought a couple of good Nicholson files from Princess auto. A single cut bastard that is 1/8" thick and 1" wide, a double cut bastard 1" wide. These were about $7 eaqch. I used a 5/16" by 3.5" grade 5 bolt and cut off the threaded part with a hack-saw. I measured 1" down from the top of the head of the bolt and marked that spot. I chucked the bolt in the drill press and lifted the table so the bolt extended through the table and the mark is flush with the table surface.
Lay the 1/8" thick file on the table, start the drill press on a low to medium speed and while the bolt is turning hold light pressure against the bolt with the file slowly moving back and forth to prevent 1 spot on the file from overheating. This will cut the groove, the final dimension for the diamater of the bolt in the groove area is .190". With a sharp file this will take about 10 minutes. Stop and measure often.
Then make sure the bolt has enough length so you can remove material above the groove to a length of .575". Use the double cut file to start to remove metal quickly but when the diameter get to about .260" switch to the single cut file for a smoother finish. The final dimension here will be .248".
The bolt will look like this at this point.
Cut off the remaining bolt above the machined surface making sure you have at least a lenght of .570" above thge groove.
With a grinder remove the six corners on the hex head of the bolt to make a roughly round head. Then chuck it back in the drill press on the .248" diameter.
Use a file held perpendicular to the table to gradually remove material from the head until it is round. I used a scrap piece of 1" square tubing to assure the file was straight at 90 degrees to the table.
To flatten and smooth both ends I just laid a peice of sandpaer on the drill press table and lowered the ends down to remove material until smooth. The final finsh was done with 600 grit sandpaper.