Sorry Steve, but I laughed!
I really know your pain! Building a boat is a lot of fun, but sanding fillets is not. Try to break up time with some other side projects like working on rudder or spars. You will get it done!
As for tools, I found a round rubber pipe cover, the type you put around pipes so they don’t freeze. This was good to put sand paper around. I cut in 3-4 inch lengths. Fits the contour of the fillet. I also watched a video of someone using a dowel the same size as the fillet with sand paper around. Same concept. One is flexible, one is not. The dowel option seems like something good to try. I also bought a small sander with triangle shaped paper to help sand.
Good luck!