Are the bottoms warped or the rims or both? Your process seems sound, and your finishing looks clean.
It doesn't sound as if you're warping them in handling, but it is easy to warp a piece like this if it is lifted before it is past leatherhard. If you are not already doing so, place a board on top of the piece in the mold, flip it, and pick the mold up off of the piece. If the center of the piece will slump when you do this, it may not be ready to unmold. If you need to accelerate the process, you can use a blow dryer to get the exposed surface of the piece to leatherhard while it still sits in the mold. If it is just a little too wet, you can use a small piece of foam to support the center of the form during the flip. Once it is out of the mold, sandwich between two boards to flip upright. Use this "pancake and flip" method until the form is too dry to warp.
What are they sitting on when drying under plastic? It needs to be an absorbent surface (I use drywall boards for forms like this). But if that surface becomes too damp, it will actually slow the drying of the bottom which can lead to warping on a flat, wide form. To avoid this, slide the piece to a dry surface that will allow the bottom to continue to dry along with the rest of the form. A single sheet of newspaper under the form will help with the moves (too much paper can wrinkle and mar the piece).
Two days to bone dry! Pretty fast. Rims can warp if they dry faster in one area such as near the edge of the plastic. You might need to wrap in more layers of plastic. I have students dry each piece like this on its own board with two layers of plastic wrapped completely over the piece and under the edges of the board. It does slow down the process, but they experience very little warping.