Heating a tube of aluminium electrically
Because your tube temperature is expected to be less than 100 C, resistive heating should be quite simple. Use a single wrap of something like Sil-pad silicone sheet around the tube, then wind your resistive coil. Then another layer of Silpad, and finally a layer of thermal insulation. You can improve performance by going with heater ribbon rather than standard wire, since this will increase contact surface area.

The inner Sil-pad layer will do two things. First, it will electrically insulate the windings from each other and from the tube. Secondly, it will improve thermal coupling from the wire to the tube. The outer layer will serve to help immobilize the windings, and the layer of insulation will increase heater efficiency.

This approach won't work for temps much above 100 C, since the maximum temp for the stuff is about 180 C. YMMV, of course, since you haven't specified the heat flux you need into the tube.