That method works quite well Alberto, but it all depends on the outer dimensions of the original kit part, as this will govern the final profile of the new, moulded part. The thickness of the clear sheet used has also to be taken into account, to avoid an over scale moulding, or an under-sized moulding. Of course, it's fairly easy to carve a balsa male mould, using the kit part outline as a pattern. The method, and the way of sealing the balsa mould, is shown in my Mosquito thread.