[How do I?] Round Nose D-bit Reamer

I need to make a 1.25" diameter hemispherical hole in 6061. It is a one-of part to replace a broken plastic bracket from a monitor mount. I have a rotab, a bench-top mill which can be tilted to 45-degrees and fly cutters. That should work but looks a bit difficult to set up accurately and there may be some conflict with a ledge on the part. I think it would be easier if I had a knee mill but I don't.

I'm too cheap to buy a 1.25" ball nose end mill for this project. I thought making a round nose D-bit reamer might be easier. I could step drill the hole and finish with the reamer. I was thinking about making the reamer out of some scrap 1018 I have laying around. Would I need to heat treat the 1018 for a single hole in 6061? Do you think this would even work? Are there any other decent options besides the fly cutter at 45 degrees?

The picture is of the part after it was glued back together where it split at the 90 degreee edge. I will make it thicker there just in case. It won't be as pretty and curvey but it will work.

What are the chances you'll need to make more of them?

None.

I'd make that cutter out of 1018 in a second! Of course, I'd treat the cutting lip with Kasenit (Yes, I've got a can! ) or some other surface hardening compound. I'd also rough out all the material I could with HSS cutters to minimize load on the 1018 tool, but it should work out just fine.

Since Kasenit is no longer available, anybody try Brownells Case Hardening Compound? The other option is Cherry Red. I also read that you can heat the part red hot and coat with sugar and quench repeating a couple of times for penetration. Anyone have luck with that?

Part came out reasonably well. The hemispherical bore isn't as smooth as I would have liked because I made some initial overcuts.

Looks good