Required:

• 5mm Coreflute/Corex/or equivalent poster board (Bunnings, or repurpose a sign)
• Closed cell flexible foam sheet (6-10mm thick), often used as postage packing
• Contact adhesive
• "Stanley"/box cutter knife

Build steps. 

• Cut a Coreflute rectangle to a suitable size to wrap around the bike tube.  Remember, the box sections of the Coreflute run along your bike frame, not around it!
• Using the steel ruler to guide the (Stanley Knife/BoxCutter) blade, slit every square box on one side of the Coreflute for the full length of the rectangle, trying to keep to the middle of the box section and not cutting through the bottom. You should end up with a sheet which is now fairly flexible.  And hopefully the same number of fingers you started with.
  HINT:  If you're fussy about making it nice and neat, use a marker to put index marks on the sides of the coreflute in the middle of each box section to act as a guide for laying your ruler down. (See the pictures for an example).
• Apply contact adhesive to the side of the Coreflute with no slits in it and apply the closed cell foam.
• Wrap the Coreflute and foam around some object similar in diameter to your bike tubing and use some tape or rubber bands to hold it in position till the glue sets.  This is to give it a cylindrical shape
• Remove and trim any loose bits of foam (watch those remaining fingers!) .  The padding should be ready to use!