I use common "Allen" wrenches -- those six-sided, L shaped jobbies made to fit hexagonal socket-head machine screws -- as mandrels. Simply get the size(s) which fit snugly inside your chosen tube(s). The six-sided contour of the wrench shaft gives purchase on the inside of the tube and helps prevent it from turning around on the mandrel as you are dressing your tube. Remember! -- lubricate the mandrel with a coating of parafin wax before the first use. That way, as the tying thread causes the tube -- plastic variety -- to crush a bit and tighten to the mandrel, it will slide off easily when finished due to the lubrication of the wax base.
The mandrels I use for my copper tubes are the appropriate sized spring steel tip cleaner -- the kind used to clean the holes in the tip of an acetylene cutting torch. (available at most hardware stores) Those mandrels have a set of spiral ridges formed into the shaft, and that's what holds the copper tubes securely while tying. They can also be used to chamfer/bevel the end holes of the tubes to eliminate line abrasion -- no need for a tube liner.
Keep your hook in the water!