Foragist --
THANKS for your kind words! I appreciate your thoughts, and your interest!
It sounds like you are noticing a bit of "play," in your stock Minelab shaft. Like many, you state that it's nothing "awful," but noticeable. I have begun to experience just a bit of this same thing, in my stock Minelab shaft, over time. This -- the "shaft instabillity" issues -- was a huge reason for, and the primary focus of, my shaft design -- to ELIMINATE that "play" that some users experience with the EQX stock shaft, by producing a very solid/stable/stiff-feeling carbon-fiber shaft (that is also very strong, light-weight, and provides that aesthetically "high-end" sort of look/appeal).
It also sounds like, based on your comments about your attempt at counter-weighting your machine, that what you are trying to achieve is not "PERECT" balance, but instead something that would just "take away" some of the nose-heaviness. As you noted, the small coil feels very "light" to nearly everyone, and thus no "counter-balancing" seems necessary; but, with the larger coils, the nose-heaviness starts to become a bit of an issue for some. For you, and those who have expressed similar sentiments, roughly 12 oz. of counterweight (in a tube less than 4" long) would be perfect. That's roughly the amount (give or take an ounce or two, depending upon how long you extend your lower shaft, etc.) that it takes to make the "big coil" (the 15x12) feel just like swinging the very light, very manageable 6" coil.
Finally, you talk about other methods for attachment of the counterweight, and possibly the cap, as well. You mention a very good idea, with some type of a "quick-release" connector. I thought about some similar options, as well. The issue, though, is of course keeping the cost low enough. It's like I always say to my wife, when she asks "if I can do" various projects/upgrades to our home. My answer is always the same, "yes, I CAN do it; you CAN do almost anything!" The barrier, though, is always the same -- the COST! LOL!
What it boils down to, in terms of shaft design, is there is always the dilemma of "do I use an appropriate 'off the shelf'" solution, in my designs, or "do I design a custom piece/fitting/whatever." And IF I design a custom piece, the BEST/EASIEST solution in most cases -- i.e. injection-molding -- is ruled out from the get-go, as the initial cost to produce the molds themselves is prohibitive, for a small operation. Injection-molding is for production of many THOUSANDS of pieces, not hundreds. So, computerized CNC machining of each individual piece is the semi-affordable, next-best option, and is one that I utilize often. HOWEVER -- the limitation there is that only some designs can be machined. For instance, it's "do-able" to machine a threaded fitting from a block of Delrin; it is NOT "do-able," though, to produce a complex, multi-part piece such as a clamping cam lock, via CNC machining.
SO, my long-winded point is, there ARE better, more elegant, and more efficient ways to accomplish the attachment of those counterweight tubes to the shaft, and you mentioned one. So far, though, the threaded fitting is the best option I have come up with, GIVEN the limitation that I need to keep things as affordable as possible...
THANK YOU, again, for your interest. I apologize that the threads on both the connector, and the end cap, are a bit more "tedious" than what one might hope. I do hope, though, that I have the opportunity to build you a shaft system for your Equinox, in the future.
THANKS!
Steve
Maybe just the quick-lock cap, but keep the threaded mount. I'd probably leave it on all the time, but change weights on the fly. 
Not practical, but it gave me the feel for it, and I could really feel an improvement in balance!
