Our students have made all of ours with whatever ratio they decided on. We go to a local 'bearing/gear' store and let them know what we want. I would advise a fairly tight (close together) spacing of the teeth. Then figure how many inches you want the rack to move with a 1/4 or ?? turn. Then you can determine the size/teeth number for a pinion gear. You can get these for about $100 total. Then machine the two plates, ~4 spacers to hold the plates apart, a solid slide guide for the rack to slip side to side on and machine brass bushings for the steering column and the pinion gear. For the slide we have found that a very stable supported, machined piece of strong 'channel' that is as long as possible and lined with a machined piece of high density plastic works well. The first couple were fairly hard to get right but we have made at least 50 over the years and nearly all work well. On the first few we would make them and then modify them to get it right using the same rack and pinion gears so we were not out any money just time.
Mike

Mike, Would you mind posting the drawings for your rack design? Maybe a picture?