A trepanning tool would work but only if it was the correct size for the groove you wish to make.
All you have to do is like I said draw the grove to full scale two circles inner and outer dia. then draw a diameter line across the middle this line represents the top face of your cutter the side will have to ground down to clear the two circles. Actually the inner circle should clear anyway, but it doesn't hurt to give a bit more clearance. It's the outer circle that you will need to grind some clearance on the side of the tool. To help reduce chatter you can narrow the tool down so it's no wider than say a parting off tool then just move the tool across the face to get the correct groove width.
It's probably a good time to come to terms with HSS you should learn to grind basic tools there are plenty of videos online and some members here have provide special courses. unfortunately from Adelaide to Brisbane is too far away for me pop over and show you.
Do you have a boring bar? Haver a look at the shape of the cutter on the end that is how it should look. In fact depending on what boring bar you have you may be able to use it fir this job.
Or see if any of the other Brisbane members are near enough to you to be of assistance. Good luck.
