making a mould... help ?

[shoebox-airbrushing]

hi... im looking at making a mould for a gt shell for the carisma, (as all the shell makers i have emailed said no)... so im giving it a go myself... i will probably modify one of the existing carisma bodyshells... probably the bmw http://www.cmldistribution.co.uk/cml...tId=0000004922

what would i have to do to this to make it into a mold... rosco said i would have to block the wheel arches off...

what filler should i use.... how thick will it have to be...