A one piece bobbin would be complicated as it would require a 3 piece mold I think. If you can make an original and make a silicone mold of that then you can cast them out of urethane resin.
There are lots of mold making supply places around but I can't remember any of them right at the moment.
OK here's one place: https://www.smooth-on.com/howto/basics-mold-making/
Alumalite is another outfit and https://www.aeromarineproducts.com/p...y/mold-making/ is a third.