With a boot, there's a lower shell and an upper shell. The upper shell attaches to the lower at the two main pivot points (one left, one right) and the flex of the boot is actually the upper shell pivoting at the lower. To stiffen up a boot like this, you need to stop the boot from pivoting. At the rear of the boot, the upper shell and lower shell overlap. If you drill a hole (or two depending on how stiff you need it) through the boot where the two shells overlap and put a large screw & t-nut through it, you just made your boot stiffer. Your boot won't pivot but it will flex because the plastic gives. It's how an SPK, Ghost, Impact 10, RS 12, and many many other boot designs operate.