I got my hole cutter from Harbor Freight. It has about a 7/16 inch
drive, not flatted. The body is around 3/4 inch. The guide is 6mm IIRC
and requires a 6mm reamed hole. The square tool bit is easily removed
and ground. The guide is replacable.

I'd hex-mill the drive for use with a drill chuck to stabilize this
tool for heavier work.

Let's see,

4 inch hole in aluminum.

CS * 4 / D

300 * 4 / 4 = 300 rpm but it's offset, so around 200 rpm maybe.

I did a 6 inch hole in particle board with this tool in a drill press.
Harbor Freight doesn't list it in Search, but may sell it if you can
find the item number. I don't have my box any more.

Doug