A simple fix is to order a weighted butt cap that just slides over your butt cap.

We've done some custom counterbalances with pencil lead pieces fastened with epoxy and masking tape but you have to sand it to fit into the blank. Other rods I've bought brass that screws into the butt handle. Even had Batson put a counterweight on one of my rods.
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I'd Rather Be Fishing for Summer Steelhead!