These inflatable lumbar support bladders are about $25-30 each. I got mine from a site called Sports Imports. If you have seat covers they can slip underneath. With my Smittybuilt covers, a strip of Velcro loop-side sticks them to the inside of the seat cover. Place it a few inches from the seat bottom. Move it higher or lower to where you need the support. The blood-pressure-style pump hangs to the side. When you need to adjust it, give a squeeze to fill or open the valve to let out some air. In this pic my bottom seat cover is off, but when on, you can't tell the bladder is there. My last set lasted about 5 years before the rubber dried out from cold weather.

 

I like that, I'm going to have to give this a try. My back is killing me after more than an hour or so driving. My lumbar usually consists of a rolled up hoodie.