Weird Lathe Cutting Tool Question of the Day

I need to bore a .25" wide hole inside a piece of plastic--with right angle shoulders. A cross section would look like this:



The "canyon" I need to make is about .025" deep, and .25" wide.

The problem is that a standard boring tool makes a nice 90 degree cut on one end of the cut, at the far end of the bore from where you enter the tube. A reverse rake boring tool (which I made one a while back) makes a nice 90 degree cut on the near end of the cut. You can achieve the desired result by using both of these boring tools, one after the other--but that's two setups to do this.

What I find hard to believe is that someone doesn't make a tool designed to make a single .25" wide cut like I need. I tried to build something that would make a single, .25" wide cut in one operation, but it didn't work. Before I could exert enough force to bore that canyon out, there was enough force to knock the workpiece loose from the chuck. The problem is that lathe cutting tools work by exerting their force on a very, very small point--and something that cuts a .25" wide piece of plastic is not a small point at all.

I have this intuitive sense that there might be some way to combine a couple of different lathe cutting tools onto a single boring bar that would achieve the desired results, but I can't quite figure out how. Or is there a part designed to do exactly what I need? Perhaps a very fine point that would make a 90 degree cut when it entered the plastic, so that I could just advance the cutting point either direction and still end up with a 90 degree cut on both walls of the canyon?

UPDATE: A reader suggested making a cutting tool for my reverse rake boring tool holder that is about 1/8" wide with a V. This gives you two right angles, and you just move the tool 1/8" through the workpiece lengthwise. The actual cutting areas are still tiny. I'm going to give this a try.