A few years ago I had a discussion with a customer (Steve Cain) about making retractable fins for the Paris sled.  We both had the same idea of a semi circle curved piece of metal with a bolted pivot point and a second series of holes or slot that would allow the curved metal to slide down and then bolt in place.  I decided against making a commercial version of this for a couple reasons.

I was concerned that the sloped sides of the Paris sled would mean that the fins will be slanted inward and thus be prone to bending with heavy loads on a hard surface like ice. I also didn't think the tooling cost to stamp the fin out of metal could be recouped by the limited number of people who would purchase it.

Steve continued on with the concept and made a great retractable and adjustable fin that he loves. He also made a pretty slick drag brake for resting on hills.  Check out the photos