I use a slab roller. Clay is rolled out at roughly the softness at which one would throw on the wheel. Slabs are about 1/4" thick. The clay is then placed on sheets of dry wall for about an hour and left to firm up a bit to about leather hard. These slabs get flipped a couple of times so that they dry evenly on top and bottom.
The walls are anywhere from 6" to 14" long. The wall corners are beveled and cut in a trapezoid shape so that the walls angle out slightly from vertical at about 10 degrees. Corners are scored, slipped, and assembled. I use a t square to try to get all corners at 90 degrees with walls straight. There is usually a little bit of a gap (1/16") where the corners meet, and I fill them with a bit of coil.
I score and slip the bottom of the rectangle, score the slab for the pot floor. I use the t square again
before pushing the walls onto the floor. Sometimes, the pot will have a rim; sometimes not. The rims are made in one piece and look like picture frames before being attached. They are scored, slipped, etc. A coil of clay is then placed around the outside join. The clay for
the coil is necessarily softer than the leather hard walls. I use an apple corer to make drainage holes. The pot is flipped
over and feet are attached.
The pot is put under plastic and left upside down on drywall for a day to help keep the rim flat and level. The pot is flipped back on its feet so that the feet remain relatively level. After a couple of days, I'll take the plastic off. Sometimes, I will start seeing a bit of rim bowing as the pot dries. Often, one side will remain relatively straight but the other side will suck in a bit. Once the pots is bone dry, I will spray water on a mirror and move the pot back and forth on the mirror to even up the feet.
I don't have any particular place I set these pots in the kiln. I use flat kiln shelves. We fire electric
to cone 6.
I have noticed that the thinner the walls and longer the pot, the more warping I see. 14" long rectangles with 1/4" walls have quite a bit of "life" to them. I've noticed that a lot of the expensive Chinese and Japanese antique bonsai pots have some warping. Maybe unless I want to use modern mold methods, I just have to live with it?