When I decided to make a vase out of a 1.5 l Costco vodka bottle, I was delighted to find an instructable that covered that subject. That instructable only covers cutting bottles that have a round section; the Costco bottle was not only square, it was also more slender at the bottom than at the top. I will show you how to cut such bottles on a wet tile saw, and while no guarantees are given, so far my success rate has been 100% (1 out of 1).
Step 1: Measure the Bottle and Print the Rings
Measure the bottle in two places to figure out the inner diameter of the rings. Add 4 mm to the largest number - this will give you the outer diameter for both rings. I designed mine in Tinkercad and printed it in PLA on a Printrbot Simple Metal.
Step 2: Cut the Bottle
Place the bottle on the saw snugly with the blade. Clamp down a scrap of wood, plywood or particleboard, perpendicularly to the blade, ~1 cm or 1/2 inch behind that spot - this will be your fence for supporting the bottle. Turn on the saw and slowly advance the bottle until both rings touch the fence. Steadily rotate the bottle toward you until you complete the cut.
Step 3: Sanding
Using wet sandpaper and/or diamond sharpening stones, dull the edges of the cut to prevent injuries. I wasn't very patient there and stopped at the "good enough" stage, but I am sure you can do a better job!
Despite the incomplete sanding, it looks pretty nice and this is a very easy project if you have access to a wet tile saw and a 3D printer.