The OI is a great boat! But there is no comparison to it and most SOFs. If you want a soft chined hull like the OI you would have to add stringers to a traditional hard chined SOF to round it out and yes that will add weight.

If you like the boat and want more info, buy the plans. You can always make a striper and also use the forms to help in making a folder if you choose. Personnally, I don't have a preference, I like both hard and soft chine hulls. Push comes to shove I like my OI better than anything I've built so far but I would never consider trying to make it into a SOF folder. In my opinion you will never simulate an OI in a folder.

A traditional SOF usually doesn't have a seat. Most often they just have a thin pad on either side of the keel. So for the most part your sitting on the skin you can't get any lower than that. I put floor boards in my last SOF forward of the seating area to make it easier to get in and out. I will never add them again as they make trying to pump out nearly impossible. Of the boats I paddled that you sit on floor boards I didn't like them at all since it raises your Center of gravity so much higher.

It sounds like you really want a folder that being the case build a traditional SOF and add a zipper like Chris Cunningham did. That will be the lightest boat you can build.