From Left to right
 ------------------
 1)      21.5 mm White PVC with 17.0 mm sliding tube inserted
 2)      17.5 mm Tight fit insert tubing
 3)   20 x 12 mm Oval conduit (preform)    
 4)   +- 16.5 mm Mandrel curtain rod (with plastic sheath
 5)   +- 16.0 mm Mandrel curtain rod striped sliced and polished ready for resizing 17.5 mm tight fit inserts
 6)   +- 15.0 mm Mandrel curtain rod with diameter reduced.(for resizing 17.0 mm sliders )

Note: 
 The curtain rod is not exactly round so the reason for the average +-
 Which doesn't seem to affect that much the end result.