Quick afternoon project: make a quick saddle, plop it on the topper model.
My workflow for the saddle: Started as a quadball T-spline body in Fusion 360. Played with it for maybe a half hour, then converted to a BRep and exported as a STEP file. Imported that into Inventor. Made two 2D sketches for the rail sweeps, then did a 3D intersection curve on them and swept a 7mm rail profile along the resulting path (I'm still using standard saddle rails here, I know...).
The workflow isn't that bad, and the whole process didn't take more than an hour and a half... but if I had a 3D scanner I probably would have used it, at least just to get the saddle top shape.
Incidentally, I spent a little while looking around the fi'zi:k site (I fucking hate the punctuation in their name), and came across their new Volta saddle. I like the look of it, and would have copied it if I had seen it a little earlier.
Ultimately I'd like to get rid of the rails on the whole assembly and mount the saddle top directly to a redesigned topper. Stay tuned.