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For the fins I've used a little piece of filament, first cut a little piece, then with a lighter burn one of the tips, once is hot press it againt something flat and you'll create a small base as a nail, then insert the filament in the holes and repeat the procedure to the other side, this second part be careful as you'll be closer to the fin, and you do not want to burn it :)
I haven´t installed yet the spring, but any spring where the main axis will fit, will do, you've just to cut it with the piece full open, I mean with the fins full open, so when retracted it will tension the spring!
Hope this help you, please let me know!!