Cheap machinable plastic is a bit of a contradiction. You need a plastic that the binder of your composite will not stick to so likely the best would be UHMW polyethylene. Maybe an alternate choice would be Delrin but it does react with more chemicals than the UHMW. Getting well away from cheap you could consider fiber filled Teflon.
One thing I think you should consider though is thermal expansion. I believe in same cases the carbon fiber composites are cured at a highish temperature? Any plastic will expand considerably. UHMW can handle up to about 80 degree C without losing much strength but Delrin might be starting to soften a bit.