I've designed and 3D printed dozens of extensions for all kinds of cues. The biggest problem is the plastic pins that folks want. They aren't made to withstand any type of stress and will break off at the connection to the base. I strongly recommend using some other connector. I usually end up building the extension and using a butt insert for the extension they want to use [predator, cuetech, whatever]. It's just a little threaded butt cap and the receptacle is easy to design.
But, honestly, the 3D printed ones are really only useful if you don't like what's out there and they weigh about the same. Most of the people that I make them for need a bespoke solution for their cue. Like a Viking cue they want to use predator extensions on. For my personal cue, I made a longer butt sleeve with an adapter for a another extension [3" longer cue, with an extension adapter].
In terms of weight, all of them are basically 2-3 oz, so you shouldn't really be worried about that. Carbon fiber is a little lighter than aluminum, and both are a little lighter than straight plastic (but not much). The mid-balance right wood one is about the same weight.
I don't know what a Lucasi butt cap or bumper looks like, but if I were in your position, I would probably just get an insert that would allow for a carbon fiber extension. Then you could get a 4" and 8" set and have up to 12" of reach. [That's what I did and what I use].
-td
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