That is thick plastic.

My 2 cents ...

Jigsaw

Lots of drill holes drilled close together in a circle.

Drill a large hole for a hacksw blade then re-assemble blade into
hacksaw while located in hole drilled in cap.

Fret saw similar to above.

borrow a hydraulic hole punch, these are great fun

Sharpen a socket with angle grinder/bench grinder then use as a punch
with the aid of a vice and a larger socket.  I have often used the
vice with home made punches to press thru thick material.  Have often
used stanley blades to cleanly knife thru thick material with the aid
of a vice.

Chisel and hammer. Might fracture the plastic.
>
> What I need to do is knock a ~45mm hole in the front of one of the pipe
> caps.
>
> Problem 1: these are EXTREMELY thick pipe caps. The plastic walls are
> about 15mm thick!
--=20
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