Michael Ray suggested using another can as a source for a reinforcement ring in a Fosters type beer can pot.
Dan reminded us that there would be a chance of that method leaving a space for bacteria to grow and hide.
From a MYOG standpoint I would suggest using a food grade sealant applied to the ring and the inside edge of the can to "seal" out the little bugs from setting up housekeeping.

Mind you there could still be a "rough" edge where the ring ends and the sealant is at its thinnest.
FWIW Unless you are completely set up to add ridges to your can and are determined to install a MYOG reinforcing ring I think Dan has the best deal going on the Fosters type beer can pots.
http://www.woodgaz-stove.com/foster-2-cup-flat-bottom.php

As much as I like making my own gear, sometimes it just doesn't pay to tool up for a single item like a beer can cook pot.
I did soothe my MYOG ego a bit by adding my own SS wire bail to my Fosters pot that I got from Zelph's.

IMHO 1 Foster can, 1 aluminum lid, 2 plastic covers and 1 FREE Generic 1 cup capacity Ridged Flat bottom aluminum can for $17.00 + $3.00 shipping is well worth the money spent. It may also save some fingers from "sheet metal distress". ;-)
Party On,
Newton