Is 90mm the pipe diameter? Seems unusually large unless it's roof gutter downpipe.
I'm not sure how you are winding the toroid. Are you fabricating a pipe core then passing turns of wire around it, each turn going through the hole in the middle. If that is the case, you probably don't want to make the core conductive because it will appear like a shorted turn and sink the power away. Also beware of the properties of some plastic materials in the presence of high energy RF fields. Some will heat up and deform or even ignite. A crude but not necessarily accurate test is to put a small piece of plastic in a microwave oven, if it doesn't melt it should be OK, if it does, treat it with suspicion.
If making it conductive really is what you need, you could consider copper tape, it's made by 3M and sold for EMI shielding. Looks like ordinary sticky tape but is copper on one side and glue on the other, you can solder it to close up gaps between the turns.
Brian.