The doors in our caravan close like a guillotine so in order to protect the fingers of our little girl I un-holstered my trusty roll of Duck tape and set to work.
A few minutes later and the door was protected (perhaps not aesthetic improved).
In the following steps I will show you how it was done.

A few things to note:
This guard when made correctly will only help to reduce the risk of fingers being trapped in a door IT WILL NOT ELIMINATE RISK! - parents still need to remain vigilant and ensure that their children understand the dangers of playing with doors.
Duck tape will wreck your paintwork and wallpaper - if you are concerned then you may need to work out a different method of fixing

Step 1: You Will Need ...

The plastic sheet is the corrugated type of plastic you often find estate agents signs made out of.
If you don't have any (mine was some packaging from some signs I had ordered) then any other rigid but lightweight material will do.

Step 2: Measure Your Door

This is the critical step to ensuring the door guard actually works!
Open the door fully then measure across the width of the rebate (The pocket the door shuts into) in the door jamb and the door leaf on the hinge side.
You are measuring the longest distance that the guard will have to stretch to when the door is open.

If you are not sure how to measure then make a template by following the steps below:
1) cut a strip of paper about 100mm wide and 200mm long
2) fold in half
3) stick the ends to the door and frame with sticky tape
4) Keep adjusting the length of the paper till it seems to fold smoothly with the door in a similar way to the video
5) Measure the paper and use these dimensions for the finished guard

Step 3: Cut the Plastic

Make two plastic strips, each half the width measured in the previous step.
Length is a matter of judgement - I only protected up to about 1200mm as our daughter tends to put her hands below this height.
(The pieces shown here are just for a small demonstration section not a full guard)

Step 4: Make the Central Hinge

Start with the plastic strips a few millimetres apart.
Next Duck tape the two halves together (Making sure you maintain the gap between the strips)
Turn over
Duct tape down the other side

Step 5: Stick to Door

Tape the guard to the door frame with the door closed.
Next open the door and extend the guard fully.
Now tape the edge of the guard onto the door. (making sure that when the door closes the part of the door the guard ix fixed to is not hidden in the rebate of the frame.