If youre set on repairing those holds and widen the tray more, you could always saw off the holds but keep the side walls so you just have a flat surface on each side of the tray. Then instead of those glassed in holds, you could run bob holds or straps after some reinforcement.
Just dont get rid of the sidewall completely, like the others said, those sides actually protect your feet from getting swept off the tray in surf or when youre in a hard corner.