I'm guessing that your pain might be more about carrying the tank out of the water, rather than in.
Spitballin, but I wonder if a taller HP120 might be better than the shorter, fatter HP117. If you are longer torsoed and mount your tank high like I do, that keeps the weight high and you need to either lean forward at the waist, or pull your shoulders forward to keep balanced, which may be working the mid spine and musculature. The longer 120 drops the CG a little lower and tends to stabilize on nature's anti-roll guide, your fundament, i.e. your butt. Your personal geometry may work otherwise - I wouldn't buy until I try.
On the other hand if it is a compression sensitivity, getting the load down to the hips should be very helpful. A rigid plate can certainly help there.
After a major blowout of my L3-L4 disc, I'm going to be probing these issues shortly myself.