I've used an early model Finisher blind for decades. It provided pretty decent back support though the rear support bar was a little high for best comfort. Later models have the bar curved downward which allowed the head to lay back which helped with comfort.
If one needed a firmer support for the back they could do as a buddy did for his dad. He took a piece of plywood and added a board on a hinge to the one end to make a ramp. A chaise lounge cushion was used for additional comfort. The hinge allowed the angle to be adjusted somewhat. A strip of Velcro from the board to the plywood as well as resting the board on the blind bag was used to hold the angle. It seemed to work well for them. I use a similar board set up in nice weather when I don't want to drag the blind around. Pull some stalks over your body and you blend in quickly and cheaply.
The greatest problem for me and my back problems was not the back rest but the recoil and upper body movement as one has only that part of the body above the waist to move with the recoil. Being able to stand or even shoot while kneeling helped considerably to alleviate the effects of recoil.
Another suggestion to look for is to buy a blind that has rather gentle or sloping angles. Blinds that have sharp angles tend to cast shadows when sunny and I have seen wary birds shy away from them. Assuming you are hunting in Texas, this can be a concern, particularly if hunting snows and/or cranes.
