I discovered my walnut pieces also have some bowing in them. I could ignore it and hope for the best, or I could establish a flat surface on both sides of each. The simple way to do this would be with a large planer, but I do not have one. I can do a credible job with a rotary planer attachment on my radial arm saw. The photo shows the rotary planer attachment on my saw with the motor shaft moved to the vertical position. I used a fence and two stops to keep the work piece locked down. The fence and the stops are slightly less in thickness than the work piece. The bright green lines show where the ends of the work piece are secured. The blue line shows how the fence secures the work piece. I am pulling the planer across the width of the work pieces while holding them down with one hand.
On the right front side of the work piece you can see something blue in color pushed under the corner of the work piece. This is a piece of cardboard from a 12 pack of canned soft drink. When I pressed on the corners of the work piece I discovered this corner rises up from the work table. I folded a piece of thin cardboard and pushed it under the corner as a shim until the work piece could not be rocked. Then I began making light cuts with the rotary planer while holding the work piece down firmly. After each cut I moved the saw arm, locked it down, and pulled the planer over the workpiece again. This process will shoot a relatively flat and true surface on the top of the work piece. The rotary planer does leave marks and swirls that will need to be removed later.