I have a bum right knee and it hits the center consule upright edge right under that kneecap. I wear a wrestler's elastic stocking type knee-pad under my pants. I got it to pad the knee, but discovered the elastic stocking it's mounted on is just a perfect support for the knee. Now I wear the damned things almost every day and have three because they do hold moisture and need to dry out more than one day.
There are many after market seats that will fit the boxster. Google it and take your time looking and measuring. Pepboys in Jacksonville has several crappy ones I don't suggest, but they are very valuable for seeing how you fit them and then measuring them to get a better idea what you like the best.
Recaro, Corbeau, APC, Sparco, etc.
Try this site;
http://www.andysautosport.com/racing_seats.html
Believe it or not, I have a Butler Built fabricated aluminum pure racing seat that has the upper-grade upholstery kit and it is QUITE comfortable with unbelievable side support. Ugly as hell.
If I ever decide to do track days, you can bet I'll fab some mounts to put it in the box.
Speaking of fabbing mounts, you may want to do that instead of mounting on the adjustment rack. It may work doing this with the stock seat, as it will lower it in the car. As long as you're the only driver, it may be the best solution.