I had got those for a couple of ten weight saltwater fly rods I was building for albie fishing. Way overkill for freshwater and don't fit in tube-type rod holders. Since this was easy to do I plan to order some more appropriate butts (and the tape to build up the arbors). It will only take about ten minutes to swap it out.
I have a couple I am thinking about shortening. I may just turn and bore a little piece of black locus. That should hold up without rotting and I have a chunk lying around. In the yak I am building, the rod storage is going to be a little over 6' so I need to get a couple of "seafaring" rods that size (paddling through breakers is the only time I will have to store rods below).
I ordered some of the tapered composite butts from Jann's Netcraft. I'm thinking instead of building up the diameter of the blank with the masking tape I might try the weight tape that golfers use to balance their clubs.
Cool ideas, I like them. My only question would be if the lead tape might shift around under heavy pressure, I guess you'll find out.
Anyway, I had an idea of using washers as the counterweight. I would insert a stack of washers on the blank between the end of the cork and the installation of the butt. I haven't tried it but it would be very easy to dry fit and see if it was worthwhile. Might look cool as well. The challenge would be to find washers that are the same diameter as the handle.