Hi moontooth,

One method that can be good for a rounded cap is to actually model the cap itself as an independent surface, and then boolean and fillet that with your base piece.

In order to do that, you would kind of extend the base piece to be taller and continue kind of straight for a while rather than trying to manually model what the filleted pieces will be later.

To give you an idea, something like the following.

I first tried to stretch your shape upwards and taper it in just a bit (not too much, you still want the fillet to do most of the rounding work, but you can kind of continue to shrink the shape downwards).

Then I drew 2 curves at 90 degree angles to one another:



I used Construct / Sweep to sweep one curve as a profile along the other one as the rail path, to make a rounded cap surface like this:



Now solidify the base piece by selecting it and running Construct / Planar to cap off its planar open ends. Then select the base piece, so that looks like this:



Now do a boolean difference, and use the rounded cap as the cutting object. That will slice the base piece into 2 pieces, select the top piece:



And then delete it, now you can select the top edges or the top face, and do a fillet to smooth out the juncture between the rounded cap and the main body:




This one was done quickly and roughly to just give you the idea, but you can mess quite a bit with the shape of the sweep curves to control the shape of the cap piece, and it also helps if you have kind of planned in advance for the main shape to extend itself upwards a bit more naturally rather than having it stop so suddenly.

Hope this helps!

- Michael