And so the box was opened.....
....Actually it was a bloody nightmare and I ended up having to hacksaw the box from the outside of the mould. Free of the box, the mould opened easy enough but the second pour of resin had clearly created some gases (I wasn't able to smell anything at the time due to the respirator) resulting in big bubbles. Unfortunately the master broke as I removed it from the bigger of the two mould halves.

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I had a try with one half of the mould and the hot glue. The resin can't cope with the temp of the nozzle of the glue gun and contact results in damage and welding of the glue to the mould. Also the mould needs a brush of petroleum jelly as a release agent. Anyways, one aside of the cloak was made from the hot glue, though it is seriously imperfect as the glue was just dribbled into one half rather than inserted between the two halves under pressure:

Though far from perfect, the result is good enough to continue to the next step of this part of the project, which is to repair the damaged half of the mould and set both halves in a new mould box, using greenstuff. After that I will drill the various in/out holes and attempt a pressure cast