No new unit - there's a place for two delrin pieces that fit into the track. The combination that seems most usual is one ball and one slug/slider. Depending on what you want, you could swap the delrin bits for two balls for less friction, or two slugs for more. It's just a matter of getting the bits and taking things apart.

Supposedly the idea was to be able to sheet without causing unwanted movement in the traveler.

I ran one slug one ball. If there was any load on the sheeting system you couldn't travel out easy without first unloading the sheet some. Hearing people talk about playing the traveler on other boats was fun - it was never an option on my 18.

I often thought about putting a track on top of the crossbeam. I think it would work fine. And, because you could always just use the old track and car, you could easily revert to class legal.

I have to admit that on the 20, I have to get used to the free movement of the traveler. That, and the cleat is on the traveler instead of fixed...