How exact are we talkin' here? within a couple of millimeters of cylindricity? or just sommet approximately round?. If the latter....
Just guessin here, but if you fold a piece of paper over (into four leafs) into an exact(ish) square and manage to draw roughly one quarter of the circle correctly - cut it out whilst folded and open the paper up. Dunno if it will turn out good enough though.
***If you dont have a compass, tie a piece of string to your pencil and pin the string down in the center. Score it with a knife if you dont have scissors and push it out.
Been watchin too much HomeFront and groundforce me thinks!...
I dunno otherwise........you sure its not 12" Dia you want
pint glass,plates etc heheh.
See ya,
sirius.