[edited from sales email]
One thing to consider is putting the colors above the buttons rather than (or in addition to) on them. The eager subject sits with hands poised over the buttons, but then has to move his fingers to see the colors. This change might produce an overall shift in RT, so you would want some field testing and consultation on this, I believe.
For those of us just doing STROOP, and thus only needing the four color buttons, you might consider a smaller 4-button box, with the buttons positioned to be under the four fingers of one hand. It's just an idea; I don't know how well it would work.