My phone is an HTC Evo 4G with an extended back for a larger battery (which the Evo definitely needs). I used a PET-plastic Snapple bottle, but I suppose almost any 16-20 oz. clear plastic beverage bottle should work. The Snapple bottle is ridged for rigidity and it has an indentation in its base. Those features were helpful.
(1) Using a utility knife, I cut the bottle around its circumference, at a height of 2.5 inches from the base, creating a small cup.
(2) I created two slots directly opposite each other, on either side of the cup rim, each as wide as the thickness of my phone. That is, I made four equal cuts straight down from the rim of the cup, using scissors, and then trimmed away the excess.
(3) I scissored off 1.0 inch of the height of one side of the slotted cup, in order to give the camera lens clearance.
(4) Using a hot-glue-gun, I cemented a tripod nut into the indentation at the base of the cup/bottle. (Various other polymer or epoxy glues might work as well.)
(5) I gently ran a soldering iron along all the cut plastic edges, to smooth them out.
My holder was finished, ready for threading onto any standard tripod and ready to accept and grip my phone/camera. I can tilt my tripod head forward virtually 90 degrees without risk of the camera's slipping out of its holder.