Make:

A beautiful result but *way* too much work. It would have been faster, cheaper and less wasteful to create a fiberglass plug mold rather than casting concrete, then creating the concave surface from that. They've only been building boats this way for half a century now.

There's a good basic description at:

http://www.valsparcomposites.com/moldmakerepair/moldmaking&repair.pdf

As an aside, it's always amusing how much dirty technology (polyester fiberglass resin, self-adhesive metalized polyester film etc.) goes into "green" products.

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I skimmed that document, previous commenter, and it describes making the plug from wood. I don't have the skills or tools to make a parabola or anything close out of wood, but cement is pretty easy, given a mold. I could imagine suspending a flexible material to create a catenary shape with a simple rig, and then fiberglass or something over that. But the cement round still seems the easiest and most long-lasting.

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(And by long-lasting, I meant reusable. If you're going to make the mirrors that small, you're probably going to need a lot of them. And then you could sell the mold or pass it around a village in need of such contraptions for cooking etc.)

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Keep it simple... why not just polish the drum?

Fiberglass is nasty, and concrete creates like 1000 tons of CO2 from burning natural gas for every ton on concrete.

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My god, what a LOT of work for such mediocre results! If it's for solar heating/cooking, why not just bulid a multi-panel fold up model that so many people make out of mylar backed plastic mirrors?

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Just stretch mylar across one end of a cylinder, seal around the edges and begin to evacuate the cylinder.
hey presto! An adjustable parabolic mirror.

Or like Dax said just polish the drum end.

Interestingly you can polish the domed base end of a coke can using chocolate (works as a v. fine abrasive!) and use that to start a fire :-)

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