If you can't find the center tubes from carpet rolls, PVC sewage pipe works extremely well. Not the "schedule 40, etc.." pipe - that's too heavy and thick, and shipping will cost a ton. I'm talking about the "sewage pipe" - I normally buy the long pieces - I can't remember if they are 8' or 10' - I'm thinking 10', and two of the sewage pipe caps (also thinner and lighter), and I spend less than $10 for the whole setup. Pack the insides and around the bow, slip on the caps and tape them in place with packing tape. I originally started buying these to make lightweight, fully enclosed, bottom side access back quivers. After getting them to my house, I found they make excellent shipping containers for longbows and custom arrows. I get the 4" diameter normally, but I think it's also available in 6".
All the best,
Glenn