I would prefer something a little more comfortable stuck in my gut when battling king salmon with casting or spinning rods than the little black plastic cap- but I don't understand what it is I should use. I am familiar with the detachable fighting butts for fly rods, but I'm not sure how you would attach such a thing to a casting or spinner?

Pesco Ergo Sum

Batson and Pac Bay both make an aluminum fighting butt better designed from casting and spinning rods. They look like the caps on the Sage Gear and longer Lamiglas rods. Don't remember the Part# for the Batson, but the Pac Bays come in different colors and the part number is AF-2 (color) in their catalog.

Check the rubber/vinyl butt caps in Mudhole.com. Looks to me like you could slip one on to try it out, and if you liked it, fasten it on or get another about the same size that could be permanently attached to your rod similar to fly rod fighting butts. To permanently attach a cork one and not make the butt longer you would have to remove some cork from the butt end, amount to be determined by the dimensions of the one you want to attach, then epoxy on the new one. Then sand to make the junction line look good. Adding a contrasting cork ring would camoflage the junction to some degree.

To attach a cork one with the butt getting longer would require that you remove just the end piece to expose the blank, then fit a piece of old rod blank of the right size inside the blank and inside the butt cap, and glue the whole thing in. I think you would want about 3 inches of penetration into the blank to spread the load, but I have no data or experience doing this. Winding the blank with guide thread and epoxying that before installing the rest of the device would provide a lot of insurance against failing the rod blank at the butt capp junction.

Now that I have given some advice you will probably get more responses with better ideas.