Sit the tube lengthwise into a slot (in a pice of wood maybe) so it cant roll around, lay a steel rule lengthwise along it so that is is "held" & run a craft knife over the tube against the straight edge of the rule - you may have to do this several time's to get a nice clean cut.
Sanding one half away would also be possible, but it might be difficult to get everything straight & even.
If you have one at hand & are VERY carefull, you could also run it over the face of a sharp plane, like a block plain - but if you slip you could loose the skin of the tips of your fingers!!