Apologies for metalworking content.....

I'm still futzing with my project to make a LED flashlight that has
the features I want but have not found in a commercial offering.

I want the body to house 3 AA cells (any chemistry) side-by-side in a
triangular configuration. I wondered if round aluminum tubing could
be thus shaped simply by forcing a tool of suitable form into the
tube, leaving the other end round for elex, 1-watt LED and optics.

I don't yet have suitably-sized tubing for a triad of AA cells, but I
tried the idea with a bit of scrap tubing .900 ID and .053 wall
thickness. Sturdy stuff, scrap from a cut up eldercare appliance.

I calculated the diameter of three cylinders such that when arranged
side-by-side the envelope periphery would be the same as the internal
circumference of the tubing I have. Turned some rodstock to that
size, cut off three 1" pieces, welded them in triangular formation,
chucked the weldment in the lathe to cut a taper on one end, and
jammed the resulting tool down the tube. I started squeezing it with
my mill vise, thinking I'd move it to the hydraulic press if it put up
much fight, but it was an amazingly easy squeeze even with no lube.

Probably obvious to most readers here, but I thought I'd mention it.
It's an easy way to make an odd shape with aluminum tubing.