The reason fillet is having trouble on your shape is due to the sharp sort of "upsweep" shape right near the end:




That fairly sharp bend in shape near the end causes the fillet to need to mutate a lot in a short distance, things like that are generally not so good for getting good fillets.

One thing that will be particularly difficult is to get the fillet to extend properly in such a way that it intersects cleanly with the back face here:



With the fillet swooping close to being tangent to that back face it becomes difficult for it to intersect well with it.

It would probably work a lot better if the shape did not swoop so steeply in such a small area near the end there.

- Michael

The "project and get side rails" way would be like the attached.

You can now select the arcs and run sweep and use the 2 open edges (which are projected trims from the 2 lines) as the rails.

The resulting sweep will hug the surfaces and will have the shape you want at each end of it, but it won't be guaranteed to be smooth on the inside areas. Generally only fillet or blend will yield something that is a smooth connection between 2 existing surfaces.

- Michael

Thanks for explaining. Very much appreciated. I'll take a look.

I ended up using loft to create the basic shapes. Then I tweaked the originating curves to the correct side/top profiles which was great as the lofted shape adjusted instantly as I tweaked. Super for 'fine-tuning' a shape. Lastly, I deleted the loft shape, added fillets as I liked and re-lofted. I tried adding fillets with the 'live' loft but things got weird fast. Seems like the "Samardac Method" is a great one for parametric-like modeling. :-)