1 x continuous hinge.
1 x sheet of 2-3mm SS or Aluminium
1 x Length of 20mm webbing
1 x little slide door lock
fistfull of rivets
Cut the metal the required size, Rivet on the hinge. Bolt or rivet the hinge to the door card (assume its metal plated all ready and make sure its level) hold up the shelf so that its level, offer up the webbing, cut to size, rivet webbing to shelf, rivet otherend to door plate and there you have a shelf. fit one of them little door locking things to hold it up or similar.
Notes:
round off the edges of the shelf as you will ALWAYS walk into it.
Try to buy a Pre-cut piece of metal the size you require for the shelf.
inspect the webbing for wear around the rivets from time to time.
Burn the edges of the webbing to form a neat finish.
Do not overload the shelf.
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