It shrinks so no it's no good. There is a special plaster type stuff but canny remember what it's called, it's used by modellers as the mould can be made quick and used same day. No special tools etc but costs 3 times that of using fibreglass...

Best stuff to use when making moulds even if only for a one off part is gel coat and fibreglass, but even this can shrink if you do too many layers at once.