IF you have equal pressure on both sides of the bait it will track straight and swim. If it has unequal pressure it will just flutter to one side or roll.
I do like the cupping idea and where the tail meets the grub, make it thin to get more action. Then all you have to do is try different durometers (hardness) of plastic to get the action and durability that you are looking for. I guess IM saying to taper the rectangular to a round shape, where it meets the tail