I have in the past produced and sold parts that i've designed initially for my own bike, but those were complex 3d printed high value parts that couldn't be sourced anywhere else. I don't think it's going to be financially worthwhile for something like the extender.
I would need to charge more than many folks are prepared to pay for such a component just to break even.
If I had a moulding shop doing other parts in grp then it would be more likely that it could be done at a realistic price, but i'm not in that business. I'm a designer, not a manufacturer.
If there were hundreds of Rocketeers wanting one, then I could maybe get a local grp shop or vacuum-forming shop to make a production tool and have them make batches of them for me to sell on, but in reality I can't imagine there being enough demand to cover the initial set up cost.
