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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Don't they make highly reflective paint that would be suitable to, say take the drum base and spray coat it?
Or what about spinning the fiberglass backer while applying a lightweight urethane or epoxy to create a uniform and smooth finished surface before using the same approach with the paint on the fiberglass?
As far as finishing the edge, I was initially thinking metallic sheet metal tape, the high grade stuff steam fitters use, but something a little more elastic would probably be better suited to conform to the curve.
Martin has a pretty good idea with the suction style but I can't imagine the mylar being as reflective...

Also you should probably attach a backing support prior to adding the reflective surface: like using flare-headed countersunk screws into a wood block; or something that can later be used to attach some kind of ball joint or support point to make the mirror more versatile rather than propping it against something.

Wouldn't a magnifying lens be more effective for creating a sun powered boiler though?

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