I've gone through a few revisions of this design and have come up with something good enough for a first run. The total weight is 30g for 8 pieces (2 per arm). These should work with any F450 or F550 frame, although the F550 will take 12 pieces.
If you want to see some high res photos of the extensions then go here:
I will send a few sets out so please feel free to send me a direct message if you're interested. I do request that you pay for shipping and the payment must come through Paypal.
If you have any suggestions for design improvements then please feel free to share them in the comments section. Thanks!