After trying so many different fixes.....I think I've finally figured it out.
I now cut my clay from the bag lengthwise instead of from the top. Wire off about a 2 inch slab. Then work the slab top to bottom, side to side, and flip over doing the same. Also use a wall paper scrape with a sponge to work the clay and compress.
After the tray is made, dry on a piece of sheet rock, place smaller pieces of sheetrock inside the tray and weight it down with kiln posts.
Then, cover with plastic, and uncover several hours per day....go slowly!
This seems to have solved any warping. ( If your tray has feet, you will need to support the underside between the feet or it will warp.)