Plastic work, but I'll use metal tools. I need to drill a 3.5" hole at
40 deg in a 3/4" piece of plastic HDPE. Since the slant depth is about
6" and a hole saw usually only goes 1.5" I see problems. The only
solutions I have been able to imagine involve making my own hole saw out
of 3.5" steel pipe and using it in a drill press or lathe (sort of like
line boring, which I know little about).

The purpose of this drill is to make a 'flange' to adapt a (somewhat
bent) SS horizontal cylinder cistern tank to accept a 3" sch 40 pipe
containing roof runoff water, and maybe a 1.5" overflow pipe as well.
small leaks are acceptable, but the result should be bug proof and stop
all sunlight.

Ideas welcome

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