Final posting on this now:
The vacuum cleaner was destroying the 3mm fibreboard, was literally sucking it out of shape. Replaced with a fresh piece, same thing happened, lost a couple of pieces of plastic due to this.
Replaced the firbreboard with 18mm MDF. There is no way that will bend!
Got a perfect result:
The entire clay mold was completely sucked into the plastic, with a bit of jiggling the clay mold got free easily enough!
Right, time to resume the PSOne project now!!
If I am unlucky and my clay mold ever breaks (eg I drop it), I can always use a finished vacuum case (unused) as a mold for plaster to turn out a new mold!
Registered on the forum here, getting some good interest in my project: forum
Also awaiting registration confirmation to go onto TK560 forum too (also specialize in vacuum forming). Learnt a few tricks there while I was doing research for my project, so thought I would post there too.