Did you use a release medium between the fiberglass layer and the cardboard tube? If you didn't soaking the assembly in water may be the easy (and only) way to get the cardboard tube out.
If you used a release meduim like teflon or polyethylene sheeting between the layers, you may be able to warm the tube with a hair dryer and slide the cardboard out. You could try pushing it out with a tube of the same diameter.
Bruce S. Levison, NAR #69055