Yes you can radius FB's without a sled/rail system --- here's how to take it to the next level. Note there are ledges on the sanding block, the rails are set the width of your block, thickness is adjusted by the height of the rails, use screws so they can be raised and lower its position using shims or washers. Make the base wide enough so you can fasten it to the bench. This photo sample took me less than 15 minutes to make and yes it is functional.
Now bare with me a second --- if the arm powered sanding block were to be replaced with an in-line pneumatic auto body repair file, (replace the flat pad with a contoured sanding block) and the height limiters attached to it (on the sides) it could be set on the fixture held down with a bungy or spring. It would not even have to be attended, once it hits the stop rails no more material would be removed. Wish I had the time to make one.
3000 strokes per minute!
https://www.harborfreight.com/air-inlin ... 62528.html